A320 procedure:
CAUTION
The use of engine bleed supply and external pneumatic power supply simultaneously is prohibited.
Before start :
-APU BLEED OFF
The BLEED valve of the running engine reopens and the cross bleed valve closes.
-ENG BLEED (running engine) check ON
-ENG BLEED (receiving engine) OFF
The bleed valve of engine to be started is closed to eliminate reverse flow leakage.
-X BLEED OPEN
Cleared to start :
Confirm area is clear of obstacles.
Ensure increased power jet wake does not constitute any hazard to people or installation behind the aircraft.
Adjust thrust of supplying engine to obtain 30 psi at start air valve before start initiation and at least 25 psi during start.
Do not exceed 80 % N2 to limit jet wake.
Apply the normal engine start procedure.
After start :
-THRUST LEVER (supplying engine) IDLE
-X BLEED AUTO
-ENG BLEED (receiving engine) ON
-PACKS Check ON
Essentially lets say APU or APU bleed failure. Start an engine at the gate from the high pressure ground air supply. Remove the connection, push back and making sure the extra jet blast isn't gonna hurt anyone/anything go ahead with the cross bleed procedure.... this includes requesting engine start at the gate and cross bleed start with the ground.
Cheers