I'm currently using some IRLB8721 MOSFETs off three GPIOs to control a 12v led strip. The 12v supply's ground is connected to the Pi's ground.
Sort of like this, but using a Pi (this is the clearest wiring diagram I could find):
If I want to add into the mix another set of lights, controlled by another MOSFET on another GPIO pin which need another independant 5v supply can I just connect that supply's ground to the existing grounds?
I think that should be OK - but don't want to get myself into trouble!