There is only one advantage and that is if you book via the call centre. With two separate one-way tickets, you pay €40 booking fees but only €20 for a return ticket. In my view, more will be achieved through a friendly talk with call centre staff than by threats. Supervisors can readily sort out problems when there is a clear case for reinterpreting rules that are not appropriate for specific unanticipated circumstances. But once you raise your voice and start issuing threats, the cause is lost.

But also for cancellation. If it is one ticket, you get one times 20 euro.

I think you mean something different than what you actually wrote...
"Funny" that Aegean would "allow" you to only pay the service fee once if booking 2 tickets (outbound/inbound) in one go for yourself, but not when booking 2 one-way tickets (or more) in one go but for multiple people... When I once had to cancel 2 one-way award tickets booked in a single process for myself +wife they told me "it's a very strickt per ticket rule". Well I guess 1 ticket number equals 1 "ticket", even if 'return', so the fact that you can't cancel "half a ticket" actually makes sense (still... in the logic of "restrict as much as you can").
Regarding the rest of your comment that's what I meant in my last sentence basically... try again (yes, politely) because you're getting multiple tickets booked by the same airline screwed up because of one cancellation. I still disagree that "the cause is lost" though. Passengers do have rights! If a reasonable re-routing is offered, then the airline would be (in my eyes) entitled to ask you to pay the fees, should you still want to cancel everything). But in this case nothing was offered.

Which ones? I've never heard of any public, free legal services in Greece!

Here's one example.
also regarding airlines
and you could file a report with other authorities as well.

As expected.

@giorginho: the link you provided refers to an, indeed, public and free institution, namely the Greek Ombudsman! Their office could certainly interfere in cases [with] a special emphasis on monitoring and promoting the implementation of the principle of equal treatment, the rights of the child and the rights of vulnerable groups (from their home page), but I doubt that T&C applied (or misapplied) by a FF program would legally qualify as an infringement on the rights of an individual for the Ombudsman to take up the matter...

This was never a matter of T&C's (!!), but rather financial damage sustained by a passenger/customer, due to a company's one-sided action that changes an existing contract between 2 parties and affecting the conditions under which more contracts between the same parties were signed. This should be self-explanatory, so I won't get into it any further.

It's not for me, sorry if I'm so stupid.

On Friday 31/01/2020 i had an email from A3 that my KWL-ICN flight cancelled by OZ. I call A3.
A3 gave me alternative flights: 1.WAW-PEK-KWL with 8 hours layover in PEK [CA738 and CA1741] instead of WAW-ICN-KWL. 2.KWL-GDU-ICN [CA424 andCA401] with 7 hours layover in GDU instead of KWL-ICN. Or to ask from A3 to cancel these two flights without penalty. I remind to her my request. All 3 flights must cancel without penalty. The {very polite} agent asks me time to communicate with an other department and she calls me on Monday.
Today Monday the agent told me that A3 will cancel all my 3 flights without penalty.
.@ giorginho: thanks for the comment to my post.

Have A3 been willing to reroute because of coronavirus waivers? I have PEK-WAW-FRA on LO which does not exist anymore. On the previous day, there is NRT-WAW. The problem is, there's no space on WAW-FRA on any day except what I have booked now. Even if they'd allow a change to NRT, that would create a >24hr connection in WAW, which is OK for me, but not sure is OK for them.

Anyone know what they would do? Or am I totally out of luck?

In the usual case of cancellation, you want to cancel the entire trip - it is less common to want to turn a return into a one-way ticket.

If your "return" is booked as two one-ways, that means you end up paying 2 cancellation fees.

I tried earlier this year to book a return but, for whatever reason, they couldn't put the two journeys on a single ticket (the outbound was on United, the inbound on TAP). They had to be booked as one ways. I wasn't happy with that - as the reason I was using miles was because I knew there would be a very good chance I would have to cancel the trip shortly before departure; which is what I had to finally do, and I did have to eat two cancellation fees.

In the ticket referenced in the previous post, United reduced the frequency of their daily AMS-IAD flight to 4 weekly, cancelling service on the day I was booked to travel,and I was proactively offered, by email, a replacement AMS-EWR-IAD on United, even though this routing was not available to book via the A3 online tool.

If your flight has been impacted by a cancellation, you can expect to hear from A3 in due course.

However, now I see you are not rerouting, but rather changing the origin/destination....that's an entirely different matter. I would expect them to allow you to cancel your existing ticket for free, but if you now want a ticket from Japan, that will be an entire, brand new ticket, for whichtehy will not be required to make any special effort to accomodate you, as it is entirely different than the ticket you hold that has been impacted.

Yeah I got that about the 2x fee a few posts ago.
However forgive me but I can't refrain from criticising one thing from your post: The low cancellation fee (and the ability to cancel up to the latest minute and even after that!) is a very generous feature when comparing to such policies of other airlines, which I guess makes up for other short-comings when redeeming with M&B. BUT: It seems to me that this service is being over-used and even abused, when I am reading your comment: "I used miles, because I would probably not fly at all"! Sure it is within your rights to do whatever the program allows you to do, but booking tickets just to reserve the award space just in case you do end up feeling like flying when the time comes... I don't know what to say... If this is common practice then in my opinion you should end up "eating" much more than 40€ in cancellation fees. I never book anything I am not 100% sure at the time of booking that I will end up flying, in fact prior to last summer, when some unforeseen and quite serious things happened that affected my travel plans, so that I had to cancel one whole trip (partly booked with cash, so extremely low refund there, but it was the cheapest fare possible anyway) and change the date on another (which due to zero award space by the time I had to cancel re-book with an intermediate stop) I had never cancelled any flight tickets ever (award or not regardless)! And the mere fact that I could (among others) cancel 3 award tickets, all booked as single one-ways and pay 3x20€ instead of losing my miles (or money) altogether (or paying high fees) made me feel grateful, not "unhappy".
Oh and by the way, I actually did end up cancelling and changing part of one trip and not the whole thing! What if you book both ways on one go and you have to cancel the return part after you take the first flight?! Is that flexibility not worth the extra 20 Euros? But... here you are, booking based primarily on cancellation fees of 20 Euros a piece...
Anyway my (only) issue with "tactics" like yours is that award space on either Aegean flights themselves or Star Alliance carriers via M&B is steadily decreasing and people who want to redeem and are pretty much guaranteed to fly are left with little to no options. If I were to accumulate a couple million miles and then book pretty much a whole "season" worth of FRA(or LHR, whatever)-HND-LAX flights in F, then cancel something like 50 flights and take the one that best suits me when the time comes, I would have secured a "flex" F sweet-spot^100 ticket for about 1000€ and at the same time caused everyone else's "dreams" to shatter...
Or to put it into a better perspective (since the aforementioned isn't happening): What if every "big" company that has personnel flown around the world constantly hired someone with the sole purpose of using miles accumulated in accounts set up for the employees but managed by the company, in order to book business-related flights in advance as award tickets...! That employee would be making (or saving) the company a lot of money, but at the same time eliminating award space completely. Perhaps people are already doing that, which could be why quite a few airlines already moved to dynamic award pricing (which I would probably also welcome, if it meant that every seat could be booked with miles).
Again, I referred to your post specifically (maybe this was a one-time "incident" on your side and I totally jumped to conclusions, even though you've given me grounds to do so) but criticised a "tactic" that I feel many with miles in the "bank" use, in order to reserve all the "goods" for themselves. Just book what you actually, seriously plan to fly (I wish I were as flexible as people who book those "360-days-in-advance" award tickets all the time and actually do fly them!). If unsure, just wait until you are sure (and then possibly pay for a "normal" ticket).

I never said it wasn't a "very generous feature"; and I certainly agree with you that it is; however, not being able to book a return for no good reason (the phone agents couldn't even book it, and had no explanation as to why not) is what I was complaining about.

Paying a fee twice - needlessly - is not nice, regardless of how low the fee is (or how late you can choose to subject yourself to that fee).

Relax - this is the first time I have ever cancelled an Aegean redemption. And I wasn't "speculatively" using up award space; I had to cancel a longstanding annual trip this year, for the first time ever. When parents are elderly and in failing health, you learn to be more cautious about making commitments that take you far away. As things turned out, I had to take the entire month of January off work.

Well, I do think you have been rather harsh in your judgement; in any event, I can tell you that there was no availability for my trip at the time I cancelled, so, if anything, I opened up space for someone else to use, and did so as soon as it was clear I could not travel.