I think you can only have one engine at a time connected to your device. If you drop it, and don't have a cab-1 or cab-2 or DCS controller addressing it (which you can have both simultaneously), it should stop. You can have multiple devices if you want.
I'm not sure if there is a limit as to how many engines Bluetooth can recognize. I have nine at the moment, but I only tried the app on the first three. Actually, it was my wife who ran them with her phone. I stick to the old cab-1's.