Couldn't find an answer to this.
I'm booking some flights for my parents.
My mother is staying for almost 5 weeks for some spread out family functions, and my father is flying home and back for the middle 2 weeks.
So, I realize I'll need 3 round trips.
Are there any restrictions to book 2 roundtrips for both of them to checkin together, and then buy a separate roundtrip to have my father fly home and back in the middle of the roundtrip?
Thanks!

Yes and no, but most likely NO. IME most (US) airline CoC's only forbid this kind of thing if it's done to avoid rules (e.g. minimum stay, Saturday night stay, etc.) on tickets.

if you booked them in a way that happens to accidentally circumvent the rules, at least do it in a way where intentional circumvention would be doubtful. say you do DFW-ORD on a thursday and return the next friday. Suddenly, an emergency pops up and you have to return to DFW on Sat for 24 hours and you buy a ticket at the last minute (or in advance if you have a personal schedule change to your activities), the airline will likely accept your explanation and wish you well.

I'm not avoiding rules, and the round-trip in the middle will have a Sat night stay. It's just to make it easier for check-ins and the such. I guess I'll have to call and ask.
Thanks all!

I did this recently with Delta/NW without a problem. I sort of had a good reason because my plans changed and the "internal" return trip was booked after the first has started though.

But regardless of reason/timing I struggle to see how the airlines would enforce rules against this, let alone try to discriminate valid and unvalid reasons. Would they cross-check earlier bookings and refuse to book a roundtrip if it is surrounding/surrounded by another trip? That seems difficult to believe.

ralfp's comment is correct. do a search for "nesting" or "nested."

Are there any other airports nearby to fly into....

... because

A-B-A with a round trip inside B-C-B is clearly allowed. (A and C = close to each other)
A-B-A with a round trip inside C-A-C is also allowed. (B and C = close to each other)

i did that once.
make sure you buy RT ticket for your parents first.
after ticketing, then, buy your dad's RT ticket with different reservation.
with different ticket number and different locator (reservation number), airline cannot tell if will be the same person.
by the way, do not give out frequent flyer number for 2nd RT ticket before check in (just in case)

Depending on the route, the airlines that serve it, and your father's frequent flyer program preferences if any, you may be able to avoid any possibility of a problem by booking the middle RT on a different airline than the outer one. (If you're booking it for him, I suspect FF programs aren't an issue.)

What if they are co-terminals?

I have done this a couple times to San Diego. A couple times a year I stay in SD for over a month, but come home mid trip for a weekend before going back. I first purchase my SEA-SAN-SEA ticket, and then I purchase my SAN-SEA-SAN ticket which takes place in the middle of my stay. Always on the same airline and never had an issue or a second glance.

I have done this when flying to Sydney as well on Qantas. LAX-SYD-LAX ticket and then booked any other trips I wanted to take once in Sydney separately. Again never an issue.

Such steps are not usually necessary. I book this kind of nested ticket every now-and-then. I have one coming up: an ORD-EWR r/t inside of an EWR-ORD r/t, both on CO with my FF# attached.

Edit to add: I also have an ORD-NRT r/t nested in the EWR-ORD r/t, and an NRT-PNI r/t nested inside the ORD-NRT r/t. All on CO. OTOH those are more a case of end-on-end ticketing, which is generally within most CoCs.

In the case of CO, the rule is written (or was as of last week) such that nested ticketing, etc. is only a violation of the CoC if the intent is to circumvent the fare rules. If the circumvention is a side effect [e.g. book XXX-YYY, (day 1 and day 10) then realize that you have to return YYY-XXX (day 3 and 4) during your stay; if this saves money over two r/t XXX-YYY (day 1-3) and XXX-YYY (day 4-10)] that's still okay.

In my experience I've never had problems with this.

Last year, I bookended a trip (2 months) with RT BAH-GVA. In the middle, I flew GVA-ORD RT, then GVA-SIN RT. In the middle of GVA-SIN I flew SIN-PEK and SIN-DPS.

I just finished doing this again with 2 RTs in Australia, which were inside another BAH-GVA RT and a ZRH-SYD-NRT RT.

I've never had any issues, have I just been incredibly lucky?

Also, great forum (first post).

Thanks for all the input.
They've changed their mind and decided to go back and forth for 2 separate round trips and fly together.
However, if they do change their mind again, I will book the separate round-trip without FF information for them just in case.
Happy Holidays to all...hoping everyone has delay-free travel.