Disclaimer: I am just reading the rules and playbook and have no actual game experience with this game. After looking at the playbook section on the cards, I believe the Union player should get to keep card #48. Card #48 says it is to be played
at the start of any Union Assault conducted across a Red Line.
Card #59 says it is to be played
at the instant the Union Player announces an Assault across a Red Line.
I would say that the intent is for Card #59's action to be played before Card #48's even if the players don't coordinate their card play perfectly. (Coordination not being a pre-requisite for wargaming)
In a perfect world, the Union player would announce his Assault and then pause for any "instant" responses before going first to add cards to his Assault as the moving player is to do. I'd interpret Card #59 as happening before any Union Cards appear, and for that reason, those cards are not actually played and should stay in the Union Player's hand or return automatically if they are inadvertently played.
That said, I'd like to see what the experts say!