JUST A THEORY BUT I THINK IT WILL WORK THIS WAY:
While holding a 1handedweapon (e.g. SMG) and standing over a second 1handedweapon (e.g. Needler) You can either:
Hold X to switch your SMG for a Needler or
Hold Y to dual wield this new neelder simultaneously with the SMG
Right you've chosen to dual wield and your now holding your two weapons (pretend these weapons are a single weapon) You can switch between weapons by tapping Y (e.g. SMG+NEEDLER ----> BATTLE RIFLE ----> SMG+NEEDLER).
Ok. I'm certain that if you press B, you will drop your left arm weapon (e.g. Needler in this case) and go back to using your SMG single wielded.
I also think that the left arm weapons wont be able to be reloaded because there is no ammo information in the top left of your HUD (see the trailers). So X will probably be used to reload the SMG alone and your needler will either be dropped or just left empty.
Thus, Dual Wielding will probably not be a permanant choice, just a temporary addition to your arsenal for the time being.
However, this is just my opinion (and i'm most likely wrong because people with be pissed off about having to drop, exchange, reload, exchange, dual just to reload their second weapon) Another theory I had was that you reload your weapons seperately with X (for right arm) and a button from the d-pad (for left) but i doubt bungie will use the d-pad as action buttons