That wouldn't work either. People would just pick their best character and never switch out because why would you? There aren't life bars (for good reason) and being forced into a second character would just suck.
This isn't the game for it. Plain and simple
Hmm.. So what if the game was based on kills then? Suppose one person has died 2 times and one more death would take him and his partner to defeat. Either the person takes his chances for a fight to win or switch out with his partner for better chances or dies. Heck, reduce it to two kills and there is bound to be some tag team switching. And plus, i'm not talking about this idea being a online mode game but a offline one.