Dealing with two FF programs
 

When I book a ticket to take advantage of the *A 55555 miles bonus, how should I deal with FF numbers? I'll fly 2 x 5 segments on different companies, so I plan to put half of them on AP and the other half on M&M.

Would it be safe to give my AP number when I book to get premium Y seats if possible, and then ask to change it to M&M when I check in? Or should I wait until I come back to mail the boarding passes?

It may be worth paying $2800 for 111110 miles, but certainly not for 55555 if somehow I couldn't credit 5 flights on M&M...

This is going to be a real test of the various res computer systems across the STAR system. Many of us will recall that last major oneworld promotion which gave 100K for flying each of five or six carriers in that alliance, and it was a real mess.

The best practice appears to be making the bookings without your FF number on the file, then mailing the boarding pass and copy of the ticket to the carrier(s) you want the segment(s) posted to.

While changing FF numbers can be done, we have had varying results reported over the years, and encountered check-in agents who cannot properly make the change required. So the "no number" approach is the safest, particularly on single tickets itineraries which will include several carriers.

If you are doing simple, return flights, or one-way, single ticketed segments, then you can use your FF number on the return portion of a round trip, or the one-way flight.

This means you may lose any pre-checkin benefits of being an elite status member, but these may be limited anyhow [i.e. pre-check-in op upgrading on overbooked flights], but it may be the safest way to ensure the flights get credited to the two or more programs you are targeting.