I went ahead and added a co-op revive.

If a player dies in co-op and the team has ran out of squad mates to switch to, the other player can revive the current player in the body of the last squad mate they controlled. I added this so the two people can continue playing together. The revived player will die if they take any damage, but they can heal with a health pack.
I hope this helps to make co-op a little more fun.