Metro Man - correct. The carts at our company have both a low and hi setting. We can use the low setting for a short period of time to run the packs, basically the (hopefully very) short time between conditioned air disconnect and engine start. It is encumbant on pilots not using APU air to operate the packs and external conditioned air at the same time.
The following is an incident report of a non return valve failure. While the report doesn't indicate the failure was because of simultaneous use of APU bleed air to operate the packs and external conditioned, I could imagine after repeated non compliance, the NRV could fail.
https://assets.digital.cabinet-offic...pdf_501105.pdf