Final answer:
When starting both engines using APU bleed air, the operating engine thrust must not exceed Idle to ensure sufficient pneumatic power and to avoid damage to the APU or engines.
Step-by-step explanation:
If both engines are to be started using APU bleed air, the operating engine thrust must not exceed Idle. This is because starting an engine with APU bleed air requires a substantial amount of pneumatic power, and exceeding idle thrust could lead to a shortage of bleed air pressure, potentially causing a failed start or even damaging the APU or engines. Ensuring the operating engine remains at idle thrust during the start process of the other engine is a standard procedure to maintain the integrity of the aircraft systems and ensure a smooth operation.