If you fire one companion, the person who joins should be able to re-hire them (at the King Crab, or in the hall of heroes if you have access to the homestead). At some point it should be possible to just transfer one companion over.
This is really a co-op game-breaker as far as I'm concerned. It's easy to ignore this problem until you find yourself having to reload frequently (for whatever reason) in a multiplayer session. Once that happens, you end up having to deal with this bug every time.
Now, it's easy to suggest dismissing the companion from the party and then recovering them at the tavern/homestead, but when you're out in the wilds adventuring and having to deal with this issue, that is a major pain in the rear.
