I'd like to somehow play music from one iOS device simultaneously on another, sort of like AirPlay, but from device to device.
It seems that by leveraging existing BT tech I should be able to make this happen, but I just haven't seen any examples nor have I read anything that suggests it's possible.
Check the following threads which might help to answer your question.
iOS Stream Audio from one iOS Device to Another
How do I stream AVAsset audio wirelessly form one iOS device to another?