I uncovered a metal junction box that was behind a wall (I'll fix this, I know it should be accessible). I looked inside and found the following: a 20-amp circuit (12 ga wire) and a separate 15-amp circuit (14 ga wire). It all looked good at first; however, then I noticed that the ground wire from the 15 amp was connected to the ground wire for the 20 amp. The circuits are not otherwise shared (hot and neutral of each circuit are separated). I think the person did this because the 15 amp circuit coming from the breaker uses a flexible metal tube with 2 wires with fabric coating inside (the house was built in 1940 or so), and the circuit was "extended" using 14-2 w/ ground to include some other receptacles and lights. Technically the metal tube would be clamped to the box and the ground wire for the 15-amp circuit attached to the box with a green screw. Should I fix this or is it OK to have the two circuits share a ground? I did a lot of searching and it looks like they can share as long as the larger gauge can handle 70% of the smaller, which it can (70% of 15 is 10.5). I asked a friend who is an electrician and he said it was ok; however I want a 2nd opinion. Thank

Its OK.

Yes, it is OK although probably not code. Remember, there is no current flowing through the ground wire unless you have a short.