Originally Posted by
Steve M
The best thing to do is to specify the FF program for the first segment at time of booking, and then at check-in for each subsequent segment, change the program and number to whatever you want to be credited for that segment. If the FF program and/or number is printed on the boarding pass, be sure that it matches what you want for that segment.
I agree with this advice. Your risk that it won't eventually work out is low, because AC/BA flights aren't valid for accrual on each other's programs (I assume). You can always call after the flights and add the miles.
A trickier situation is when the miles could go on either program, but you want to direct them to a specific one. This happened to me twice this year flying on itineraries combining QR with UA/LH/LX -- I wanted the QR miles credited to my QR account and the Star Alliance miles credited to my UA account. And, since QR is a partner with UA, I was worried about QR miles going on UA or vice-versa, because to fix that would require removing miles from the account, which I imagine is difficult. Luckily, I applied
Steve M's strategy and it worked fine.