For an unexpected encounter on the street (robbery) with 2 or more BG's I would more than likely shoot at the one I could engage the quickest with 2 shots then proceed from there to the next quickest target to engage. This would be for a "normal" robbery with the BG's all within 3 yards of each other and armed with hangguns or knives.
Reasoning: At this distance my first shot might be point shooting and I want to get as much lead in the guy as possible. Notice I did not say the closest BG I would engage first. At these distances I would consider them all equal as far as a threat and do not want to take the time to asses who is carrying what. Taking the time to sort them out would not make much of a difference so engaging any of them quickly would be first in my mind.
And of course llike the OP stated, create distance and movement while drawing your weapon.
Oh, the other reason for two shot is because I always train for two shots. It would be instinctive even if the first shot took off the guys forehead. I can see either way though, single or double.