Have a question on splitting when there is OAL on the itinerary.

Have an 016 ticket, itinerary was booked through UA for me, wife and lap infant. Revenue ticket. It's two segments each way, which I'm used to. Outbound are both UA-operated flights, but return starts with an AC segment (it's a real AC flight number - not a UA codeshare) connecting to a UA flight.

If the record splits on the outbound (for CPU..or just because), will this affect the AC flight on the return? What is supposed to happen - will the AC record also split, or is this likely to cause issues? Also, bought preferred seats with AC - does that complicate matters (I understand it's supposed to be tied to ticket..but who knows)?

Any experience one can share would help. Obviously, don't want trouble when trying to get back home.