I want to get my bathroom lights and outlets on separate circuits because the outlet is not GFCI nor 20 amp and the lights always dim when my wife uses the outlet. Everything is fed via flex metal conduit and very intertwined with other rooms, and the main panel is full. BUT there is a sub panel directly below the bathroom and the sub has a grounding conductor leading all the way back to the main.
Can I run a NM 12-2 from the sub to the outlet, and separate it from the other wires, but keep it in the same metal junction box? Or does this pose a problem having it grounded the main via the metal conduit while at the same time grounding it to the sub panel via the NM wiring?
Stay out of the existing box if you can.
Run the new circuit to a new outlet in a new location if possible. Trying to stuff a GFCI into an existing box can be tricky. Use a new plastic box screwed to a stud.
If you DO go to the existing box, keep the lighting circuit as is.