Can the ground wire from two separate circuits (one 20-amp one 15 amp) be shared?

There wasn't room for more details: I know they're shared on the bus bar. I meant to ask if they can be connected in the junction box? I found a jct box with the two circuits. They are completely separate except for the grounding conductor/ground wire, which are connected. This is because the 15-amp has armored BX and junctions with 14-2 romex, so the ground for the romex connects with the ground for the 12-2/20-amp. Thanks.