Hi Dave,
I have no idea how the NG does it, but in the Airbuses the automatic starting just closes the pack valves, so the pilot has no influence over that... Maybe the NG has a similiar feature?
On the classic (and also the 747-400 I flew, we´d start 2 engines at once with one pack still running) this was up to the pilot to do. You need a start-pressure of at least 30 PSI. If you have a cold day on a low airport, you might get away with the APU also powering the pack(s) - but it sure puts a strain on the APU (high EGT) and if the PSI´s drop too much, you can have a bad startup overtemping the engines. Note that these effects are not really modelled to a realistic extent on our 737, so it might be more forgiving than the real system.
Jan