I believe in the DC Rainmaker comments that Garmin confirm the response as you can only have 1 pair actively connected at any one time, so no you cannot have a friend listen to the music at the same time with a separate pair of headphones.
Will it be possibile to connect two headphones to the same watch at the same time and run with a friend listening to the same music?
Even the idea of one runner wearing/carrying the music player, and another wearing the audio headset connected to the player by Bluetooth, would be impractical and problematic anyway, given the generally cited range of Bluetooth under favourable conditions (10m or 33ft), which is reduced when there are physical objects (such as human bodies) getting in between the music player and the headset.
Yes I understand that the connection could be difficult, but if I wear the watch on the left wrist and the other runner runs next to me on my left, his/her head is a little farther than mine from the watch so I think it could still work.
Maybe the FR645M, being a new (ergo far more recent) model and/or of a better brand, would be much better than the TomTom Spark Cardio+Music I had in that regard. It was hard enough to get music playback not to break off intermittently, when I was wearing both the watch and the connected headset. Different headsets have their Bluetooth radio hardware in different places (some at the left ear, some at the right ear, and some behind the neck), and unless the watch was on the same side as the headset's radio, even one metre between watch and headset (with my head or body in between them, depending on my arm swing) proved a problem sometimes, especially when I ran outdoors with no walls for the radio waves to bounce off. I also have (some, but far fewer) issues playing music from my mobile handset, which is tucked into my running belt, to my Bluetooth headset.
I reckon getting someone else to wear/carry the music player while I run alongside them would simply be an exercise in frustration on my part as the listener.
