Originally Posted by
cblaisd
If this has been covered in another thread, feel free to chastise and link me
It indeed has been, and recently, too. Consider yourself thoroughly chastised, Mr. No-Search.
However, I don't have a link for you, so I'm equally bad.
According to
mahasamatman, the PNR should have space for two FF numbers - one for mileage earning and one for status-based issues (e.g. seating). I have not seen verification of this claim but I've seen the claim a couple of times.
Alternatively, you should always be able to switch your FF number at the gate, however if you do this on the outbound flight, you might need to have it switched back to MP for the return flight to ensure you're not swept out of E+... then do the same thing at the gate for your return flight.
(And miles should not get removed just because you credit the outbound to one program and the inbound to another. There's no requirement that you credit only round-trips.)