When you are starting up a 747 or 340 or something similar, I am under the impression that you can start two engines simultaneously. Is this correct? I suppose you would require an APU with an air output sufficient to spin two starters.
On the 744 the APU is powerful enough to start 2 at a time, with 1 pack running. We routinely start 3+4, then 1+2. With ground air, only 1 at a time can be started, and usually the pack has to be turned off as well.