We may play with two strikers, but I wouldn't bet on it. When has Van Gaal ever played with 2 strikers (No.9)? I don't remember.
And yes Robin is untouchable, he is an another level to Rooney, he knows Van Gaal well and is his captain in the national side. Unless Robin gets injured or does a Torres he'll start as our main striker. Rooney and Mata (and who knows, possibly Kagawa too) will fight for the No.10 positions. And unlike Moyes, Van Gaal has bollocks to bench Rooney when he plays like shit (like he did on the second half of last season). On the other side, if he plays like he did on the first half of the season (or the second half of 2010-2011) that position will be his own.