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. Thanks.
Edit: this is different from another, yet similar, question because this involves two different amp circuits, and this one involves BX cable.
Edit again: Thanks for the answers. Running a new ground from the box to the bus is an unrealistic option. Regarding another comment: The 12-ga ground wire isn't connected to the BX except for incidental contact with the box.
Since there's room, should I separate the two in separate boxes? I would ground the 14-2 to the metal box and be sure the BX is clamped good and tight.
Picture description: lower right 12-2 connects with yellow 12-2. Armored comes from bottom and connects with the two 14-2 coming out the top. (I traced the armored to the box, so it is grounded the best that stuff can be; built in 1940.) Ground wires (grounding conductor) for both are twisted together. There was a green screw in there, showing evidence that at some point the possibility that the grounds for the 14-2s were connected to the box.