What? I am not sure if I understand your question, but if you are asking why the pressure is too low: The engines deliver less bleed pressure than the APU, so after engine one is started using a huffer the external air source is disconnected and the pressure in the duct will drop to the 15-20 psi that the engine delivers. That is insufficient for a normal engine start, thats why you need a bit of thrust on engine one to start number two... Is that what you meant?