Hi Altomar,
You don’t half believe in making things difficult.
The ‘traditional’ way to get audio from two (or more) sources is to feed them into an audio mixer, which allows you to mix and balance the audio feeds, then feed the resulting ‘mix’ into your intercom and thence to your headset(s). After trying several other methods, that is what I now do in my plane.
But you say there is no input into your headsets. That is unusual - most intercoms have a way of incorporating additional audio inputs. I take it you aren’t using a Lynx intercom? They normally have options to feed external audio into the system - even if it means using an external combiner. I’m sure I have seen in-line adaptors which fit into the Lynx headset lead - have you checked their website?
PilotAware Rosetta does have audio out via Bluetooth, but you can’t ‘pair’ an earpiece to more than one source at a time (the clue is in the term ‘pair’). I would also be wary of wearing an in-ear device inside my aircraft headset due to the possibility of missing radio messages (though I have used dual earpiece headsets for emergency comms in my former life).
I have also tried using a bone contact Bluetooth headset (where the ‘speaker’ sits just in front of, rather than ‘in’ your ear) - under my normal headset inside a helmet, but found it very uncomfortable after a fairly short while and again I could only get one set of audio through this - hence why I changed to the mixer.
If you are happy using a bone-contact headset under your aviation headset, it might be possible to feed the output of a mixer into a Bluetooth Transmitter and then use the bone contact headset to pick up the combined audio, but I have never tried this.
Just a word of warning - DON’T try mixing your VHF comms audio with anything else unless you are very aware of what you are doing.
If you do manage to solve this conundrum, please let us know how you do it.
Regards
Peter