I'm finalizing my plans to make a physical control panel to interface 2 of my 3D Printers to an embedded Linux device. (Just minimal physical controls since 95% of the controlling will be done via a web browser rather with physical buttons.)
Each printer will have it's power supply controlled by the mCU, and about 20W of LED lighting.
One printer is powered by an ATX power supply that is also powering the mCU with it's +5V standby power. The other printer uses a non-ATX SMPS that has a PWR_ON pin similar to an ATX PWR_ON pin. (IOW, floating or high = PSU off, low = PSU on)
My main concern is whether simply connecting everything to a common ground is sufficient to keep each PSU and the mCU safe as the two power supplies kick on and turn off.