Duo is the go to mode IMO. I've never seen pairs of Duo's working together?
When myself or a team mate is downed, can sometimes work to your advantage as people tend to rush the downed player to finish him, only to be meet with a quick death.
Nice to have decent un spoken tactics working in pairs also... My mate I when entering a house, we'll go in together, he takes left and I take right, at least that way we aren't left with our fingers up our **** if someone is hiding in a corner.... Works even better in squads of 4... corner each!