This has been suggested many many times before.
But, it will only be buggy. Hampa tried to do it before and he said it wasn't fun at all. So...unfortunately, this probably won't happen for quite some time.
Long version: It has been tried and tested (some of you may see screenshots and the like of that time) and playtesting ruled that 2v2 was a) not very fun and b) made physics wonky, namely things were a lot slower than they were supposed to be.
Pic
as i already have an 3.X version installed and the question may come up at some point again i made a quick screenshot and .gif that it actually works.
Pic
and the .gif