[calcapt]

Doug is basically right. On the 737, you are able to isolate the #2 engine and right air conditioning pack from the rest of the pneumatic system. If you taxi out on one engine, starting the number two engine first allows you to run the right pack with the #2 engine and the left pack with the APU bleed. If you are starting them both up, starting the #2 engine first allows you to get air conditioning going right away by isolating the number two side while starting the #1 engine with APU bleed air. Taxiing out on the #1 engine would give you the option of running the left pack or the right pack, but not both. That's why you see so many who start the #2 engine first.

I always ask about late bags before starting the #2 engine as the cargo doors are on the right side of the aircraft and the #2 engine would need to be shut down to accommodate any late bags.