Is the plus rule actually in effect for the game? If so, it should work like you described it, so long as both the other cards are in the opponent's possession.

You can get limits like this:
Squall- upgrade his weapon
Rinoa- progress in story line and read Pet Pal Magazines, then walk Angelo (this is explained in the game)
Quistis- Use certain items
Zell- Read Combat something magazines
Selphie- They're just kinda all there xD you can open the lid to give you more time to toggle them
Irvine- Buy different ammo