The memory is a little rusty on the exact positions, but the basics are that on the 757, the air start is usually positioned behind the wing, so it's not such an issue as to No 1 or No 2 started first, but then the air start has to be removed before the other one is started. If there's an airbridge involved, then it's better to have the airstart on the No 2 side, for ease of access of all the other items, especially if it's a towed unit rather than one built on the back of a small truck chassis.
Ideally, push back is then done with one running, if for no other reason than a 757 with both running is not the easiest of machines to push, unless you've got a tug that normally deals with 747's! If it's a "small" aircraft tug, then both running on a 757 makes things "interesting"!
On the 737, it's a different story, due to the much lower height of the access port for the air. In this case, the airstart pipes are ahead of the wing, and that means that there's no way the operative can go past a live engine on the 737, so it's always No 1, then disconnect.
Single or dual airstart depends entirely on the capacity and condition of the air start unit, and a weak single air start on a 757 can lead to some very anxious moments in the tower when it eventually starts, as it's likely to be VERY smokey, and there's a much higher liklihood of a hot start, as the rotational speed of the engine will be marginal in comparison to the normal speed at which fuel is introduced. On one occasion, I got seconded to another handling company to do an airstart on a 757, and our airstart was not in the best of condition, so it took a LONG time for the core to spool up, and I suspect that the flight deck opened the fuel flow at slightly less than the normal rotation speed as it wasn't going to get any faster, so we were "treated" to a large cloud of very white smoke, and a very quick inquiry from the flight deck to the headset man " is everything OK, as ATC are worried we have a fire". There wasn't a fire, so they recalled the fire service who had already been dispatched, but there were some "interesting" discussions later about that start.
Almost as good as the time when they scrambled the fire trucks because of the clouds of very black smoke that the de icer was emitting
