I'm remodeling and making some minor changes to a 20 amp circuit that I intend to extend. I took out a receptacle at the end of the run and have a free piece of wire that isn't long enough to make it to my first new receptacle location. However, nearby is a large plastic, electrical box on the wall that has a sconce mounted on it and one 14/2 w/g running in to it from a switch on a separate circuit. Since there is plenty of volume in the box for more wires, and I can reach it with the end of my free wire, I'm considering bringing the end of it into this box, splicing it to another piece of 12/2, and running this new piece on to a new receptacle box from there.

I would simply be using the box for the sconce as a junction box for this separate 20 amp circuit where I wire-nutted two pieces of 12/2 together. Is there any problem from a code compliance standpoint of having these two separate circuits sharing one box as long as I don't exceed box fill requirements? Also, I presume that I just wire nut the two grounds in the 12/2 together, rather than wire nutting them to the ground for the 14/2 cable...correct?