Originally Posted by
Unionruler
In your shoes, I would put the UA number on first because lounge dragons could want to see the UA number on the BP in lieu of a physical card, and then ask the gate agent to delete or replace the FFP number.
Personally, I've had issues with this. Checked in with A3 number as required the free checked bag, proceeded to lounge etc. Got transfer desk at SIN to change the FFP number (to UA, as I have sufficient for A3 *G) and got a new boarding pass, boarded with the new boarding pass. In the end, still credited to the original FFP, which is A3.
To OP: I'd try to reason with lounge dragon that *G gets lounge access when on *A carrier, nothing says I have to credit to that *G FFP to get lounge access.
Then again, YMMV, try at your own risk.