I really like this idea! However, I see some problems...
It seems it would be better for organized game-play. Having a smaller team means you would have to put a bigger emphasis on working together. (something pubs in this community lacks...) I don't think many of the random strangers who join pubs just for fun have a keen sense of listening skills.
Also, with the TDM history I've seen... Public TDMs turn more into a giant blob of splish-splash damage around the center of the map. BUT, having the separate teams may eliminate this problem.
But again, I really like this idea. I personally believe it would be worth a test... Just to see how it goes over. This would be something new that I have not seen in many other games.