not long ago, flying JFK-FRA together with Gisela on LH, at check-in I changed my ff-program from miles&more to Mileage Plus (no change for Gisela's ff-program = miles&more).

I noticed later, that my Mileage Plus number now was on her boarding pass (and my miles&more number still on my boarding pass).

I also got the credits for that flight in both programs at the same time (We/Gisela doesn't mind, she is not participation in the fly-5 get 55'000 miles promotion in both programs), and for me it eases to maintain status in both programs ...