The bleed air sources for engine start are: APU, Ground cart or the other engine.
Using APU, normal start sequence is 2 and then 1 (Boeing recommendation). As you can see from the diagram
(http://www.b737studyguide.com/B737NGiPad/AirSystemsSGR/PneumaticSystem_800_iPad.html ) APU is connected to the left side of the bleed air system, so you must set isolation valve to open position. Be sure to follow the right steps for engine start.
The ground air cart is connected to the right side of the airplane and the right side of the bleed air system. Although it is connected to the right side of the air bleed system, engine number 1 must be started first, (boeing recommendation) for safety reasons because ground personnel and equipment are in the area of engine number 2.
The APU bleed air switch must be off. After starting number 1 and the engine is stabilized disconnect the external power equipment. Then use crossbleed air to start number 2.
Follow the procedures for crossbleed air start.