@Dream747,
I'm not familiar with the engines installed on the Airbus 320 series, so correct me if I am wrong.
The engines when running at idle supply less pressure than an APU.
Leaving the APU bleed on ,while engines on, will only keep the check-valves in the engine bleed supply ducting closed.
The apu will still be suplying all the air into the packs.
It's why an engine cross bleed start requires adding power on the engine already running to obtain sufficient start pressure.
IFIX