[lorenzhs]

Sennheiser has this figured out, too. My PXC 550 stay connected to two devices at the same time. When the currently active one goes quiet, it'll switch to the other source if that's active.

Works fine with my UE Boom 2 bluetooth speaker as well.

Maybe the devices you've tried were older or cheap models?

[freeone3000]

The Sony MDR1000X2s can only stay paired to one device, and they're neither old nor cheap. (I was aware of this when I bought them, they noise cancel better than the QC35s for voice.)