I can't speak specifically to Sena as I only use Cardo and don't own a GW (yet), but I can say that this problem isn't isolated to Sena. I think it's because when the communication units are in "Intercom" mode, it won't allow for music playing/sharing. I'm not sure what the exact solution is. Maybe connect the systems to each other first via Mesh or BT, and then connect them via BT to the bike for audio?
Best of luck, and enjoy the GW, it's a gorgeous bike!
Best of luck, and enjoy the GW, it's a gorgeous bike!