This is why most people believe that FFP's do not like pax using their elite status for lounge access, but crediting flights to another program.

Basically when you visit a lounge the FFP of the card that you show pays for your visit (Somewhere between $10 and $40). They recuperate those costs when you credit the miles of the flight(s) to that FFP - meaning the FFP receives cash from the airline. That cash is used to pay for your lounge visit, and to credit your FFP account with some earned miles.

So when you use FFP1 for lounge access, but credit the flights to FFP2 - then FFP2 receives a cash amount (payment from airline), while FFP1 loses a cash amount (payment to lounge operator). Which FFP1 would not be happy about.