Hello Ronin,
The intended function is to have the 3CX Android App ring on the device's speaker. If the bluetooth device is connected and takes over the speaker function, it will ring via the speaker (being the bluetooth device).
Just a note: On some android devices, the "Car/Bluetooth Support (Telecom API)" enables the use of the new Telecom API which may be useful to you while exploring your possibilities (outside of 3CX settings) to have it ring on two devices. The API is intended to allow bluetooth devices to pick up the calls via their available buttons.