First impression is that the guy who is bandaged up and helping "Snake" is the "real" Big Boss. It seems clear that he's with you for at least significant parts of the game, helping you sneak past guards. Note that he's doing most of the leading.
Or perhaps more to the point, both of them are Big Boss. Having divided his identity within what is likely a coma dreamworld, "two phantoms were born." Thin Snake is heavily wounded (handicapped in fact), weak, and unsure of his identity. Bandaged Snake is muscular, moves with purpose (including in BB's crouch-walk style), and perhaps knows who he is but is intentionally holding back (as does the trailer). Bandaged Snake has a very un-Snakey voice, but after he's unmasked I would suspect he would then again have his "real" voice.
It's also very possible that this isn't taking place within Big Boss' comatose state, but rather a result of a serious drug-induced state causing Big Boss to dissociate and hallucinate. Either way, I really liked this trailer and the backdoor presentation of this new MGS.