Audio Double Ended Bluetooth Audio Stream in Unity

Hello all! Right now I'm currently investigating whether a certain feature is even possible to implement. Currently, our Unity game has a Bluetooth menu that can connect to an audio output device like headphones and output the game's audio to it. This is working fine. It might be worth noting that this game is running on a custom Android tablet, but this Bluetooth functionality is working.

However, my client has requested a new, special functionality. They would like a user to be able to stream music from their phone to the game via Bluetooth, and then have the game stream the combined audio (phone's music and the game's SFX) to the headphones. Like this:

Phone -> Game -> Headphones. All three different devices.

I'm not sure I've ever heard of someone doing this with Bluetooth? This double-ended audio stream? It was my understanding that Bluetooth only really lets you do a one-way audio stream between two devices. Even if it IS possible, I'm not sure there's a good plugin for Unity that would allow it to receive the phone's music? Our custom tablet is running Android, and the only way to MAYBE do it would be to maybe have some other app running in the background that receives the phone's streamed audio. But then that app would also have to have the capability to then stream its received audio to the headphones.

I've been looking around online and haven't found many people trying to do this. Probably because it's simply a better idea to let the user just connect their phone to their headphones like a normal person and listen to their music that way. However, I told my client I'd at least look into whether or not it's possible. So, any ideas?