I can book two round trips for a cheaper price then two one ways.
aaa-bbb-aaa, bbb-aaa-bbb is cheaper than aaa-bbb, bbb-aaa. I can book the roundtrips such that the first roundtrip ends before the second roundtrip ends. Is this illegal?

You're nesting the tickets. Other threads go through in detail the legality in terms of the conditions of carriage.

If you're also throwing away the respective returns, that's throw-away ticketing.

I thought throw away ticking was a one way aaa-bbb-ccc where you didn't fly ccc.

If my case was aaa-bbb-aaa leave 1/1/2018 return 1/7/2018 first ticket; bbb-aaa-bbb leave 1/14/2018 return 1/21/2018, this would still be nesting?

No, because ticket A completes on 1/7 and ticket B starts on 1/14.

If ticket A was 1/7 - 1/21 and ticket B was 1/7-1/14 it is nested. But nesting is only a CoC violation if used to avoid pricing rules/restrictions.

As this is a general question and not specific to United, please follow this in Travel Buzz.

l'etoile
UA moderator

What definitely is a violation of the CoC is back to back ticketing, which is commonly done to avoid a Saturday night stay requirement.

I'm trying to figure out what you're trying to accomplish but to me it looks like throw-away ticketing in that you don't intend to fly the return legs of either ticket.

In other words, if you fly both legs of each ticket, you will be back in aaa on 1/7/2018 and then leaving from bbb on 1/14/2018. Is it your intention to fly aaa-bbb on 1/1 and then fly bbb-aaa on 1/14? And a round trip aaa-bbb-aaa on those two dates is also more expensive than the two round trip tickets? It seems like there's something I'm not getting here.

OP - What is the carrier (or if multiple carriers, the carriers)?

There is no industry-wide convention on prohibited practices and what a carrier considers "nested", "throw-away" or "end-on-end".

The secondary question then is, if what you propose is not permitted, what will the carrier likely do. That is again something which requires us to know the carrier.

To cut it short, do this twenty times or so and you should expect a phone call or email notification that you FF account is closed with all miles gone. Whether it's nested tickets, throwaway ticketing, end on end, or hidden city, you'll eventually get found out if it's a regular event. You're not the first bright spark.

That is hidden city ticketing.

Throw away ticketing is not using the return portion of a round trip.

The allowable way to go about this (and what E&Y used to have us do on long term assignments, back in the day), is to buy an initial one-way aaa-bbb, then buy round trips bbb-aaa so you get the Saturday night stay. Not sure if that is an option for the OP.

That's exactly what our corp TA required us to do back in the era when Saturday night stay requirements were common.

For certain markets we could buy "open ended" tickets. An AAA-BBB-AAA R/T, where the return date was flexible and simply had to be selected within 1 year with a 72-hour advance request. (Pending availability of course, but it booked into a relatively available Y bucket as I recall.) Traveling during the year from BBB (or wherever else), even back to AAA on other tickets on the same airline, was permitted. But I haven't bought one of those since 2000 or so.

Trips are on United.
I don't know exactly when I will be flying back. Just in this example, not on 1/14. Right now if I book two one ways the price is >$11,000. Two round tickets <$7,000. There is the chance that the round trip might increase more than $4,000 and I would lose money.

Not just the return flight?

More like a half dozen times over a year.

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Over the years I have ate the return flight and purchased another round trip because this is a frequent (not 20 times a year). Sometimes I would buy a one way. My upcoming travel looks like I could save the lease payment on my Lexus if I bought two round trips.
For this reason I was wondering about the 'legality' of these bookings. In particular the one I quote above. Also if I didn't book the return round trip until I canceled the original.

It sure smells like throw-away ticketing to me. "Illegal" is too strong a word - this certainly isn't illegal anywhere I'm aware of. But it quite possibly violates the airline's CoC, which means they could, in theory, try to make life uncomfortable for you in one way or another, assuming you're doing all of this on one airline.

Whether 6 times is enough, I don't know. Personally, I'd find a way to leverage two airlines and perhaps some sort of open-ended ticket type, if one exists. 6 throw-away tickets would exceed my personal risk tolerance on any airline where I cared about my miles and elite status. Which, if I were flying long-haul international J/F (which this appears to be), would be all of them.

I predict you'll get caught and UA will close down your MP account.