Two people can occupy the same space (often two people are spawned in the same spot in really large games). From an outsiders point of view, one or both will intermittantly appear invisible.
I use the same space thing to save me some damage when I'm reloading. It doesn't work if they other guy has a brain, but a lot of them don't.
Actailly that's a cool feature in the campaign mode too. Remember how in Vanguard, your ally troops were always getting the freaking way? In this game, they go kind of transluscent when you're both leaning around the same cover.