I think the reverse of this kind of exists in the from of Bluetooth to 3.5mm receivers. The one I have and most I've seen support two connections so I can have

I think the reverse of this kind of exists in the from of Bluetooth to 3.5mm receivers. The one I have and most I've seen support two connections so I can have both PC and laptop output sound on some speakers. You could probably chain a multi input receiver to a single output transmitter via 3.5mm to your Bluetooth headphones and just toggle connections on the receiver to select the device you want your headphones to use. The bad news is the latency which is about 200ms regularly and 40ms with APT Low Latency or 80ms with APT Adaptive. If you go BT > BT RX > BT TX > BT this might be doubled. Both devices will also need to support the same codes and only newer devices support APT Adaptive.