Grounding metal box joining two BX cables without ground
If this is old armored cable, commonly reffered to as "BX". Then the armor was never considered an adequate grounding conductor  (at least not that I'm aware of).  A circuit wired with "BX" would be considered an ungrounded circuit, just like any other two wire circuit.

If you want to add a ground, you'll have to install an NEC approved ground.  If you just want ground-fault protection, you can install a GFCI device to protect the circuit.

If it's modern Type AC or Type MC cable, then you'll use the bonding strip or grounding conductor as the ground.