Ignoring all of the above so that we can move on...
Well, the huffer/ASU/Apu on wheels is connected to the right side of the airplane very near engine 2 - that is the reason why it has to be removed before starting engine 2 and why engine 1 is started first. Clear?
1) ASU delivers relatively high pressure air to the right side of the bleed system
2) ASU is disconncted once engine one is running, pressure drops to the normal ground idle duct pressure
3) Engine one is throttled up to give sufficient pressure to start engine two without the help of the ASU.