How many times have you seen 2 kids sharing a mobile phone headset each with one earphone both trying to listen to a piece of music on a phone.
Simple solution would be "silent disco" mode on your phone app whereby one user is the master DJ, streaming your service and selected friends in bluetooth range can tether and piggyback "listen in" on their own phones/headset without having to be physically tied together within the wired 1 metre (and only get mono).
Updated on 2019-05-02
A similar idea has also been suggested here:
Regardless of status, please vote for it, and add a comment there, so it may be considered. Any update on the matter should be posted there.
For more info on how the Ideas Board works, click here. 🙂