Perhaps someone with more knowledge or experience can confirm that this actually works.

I know Amadeus, the base system underlying BA's IT, allows multiple FFP nos to be entered. However, I thought that there was two types of field - one for the primary FFP, which determined posting of avios for instance and another for other FFP numbers, which were in effect just note fields.

If my understanding is correct, anything in a note field will not trigger benefits such as seat selection. Other airlines may use these fields differently, but I thought BA's systems will ignore any secondary FFP no., which is why the normal advice is to put the highest status no. in first and get the benefits, then change at the airport to credit to a different card.

Of course, I may be completely wrong and if so, this would be very useful for many.