rog747,
Normal practice with a u/s APU is to crossbleed-start the second engine, as you thought. This is sometimes done during pushback, I think, although the increased thrust on the delivering engine puts a strain on whatever vehicle is being used for the pushback. Better to wait until pushback is complete and the parking brake on. If the apron is too busy, it can obviously be done before pushback, provided pier structures and ground equipment are well clear of the delivering engine.