I do not own 2 FH, but I have numerous bluetooth devices in my house.
In the FH menu, at the bottom is a Settings tab. Within that, is a "Devices" area where you can select which Device to speak to the app. My best guess is you could then select the FH of choice.
Alternatively, you can hold the unwanted FH bluetooth button down to turn off bluetooth (or flip the power switch off).
I find the bluetooth connectivity a little picky so I often have to reintialize my app (android phone) with my FH.