Question about Logitech Bluetooth Audio-Receiver

The 3.5mm socket is a switching socket, i.e. as soon as the 3.5mm plug is inserted, the cinch outputs are switched off.
This means either cinch or 3.5mm but NOT both at the same time.

You can therefore connect two devices in parallel (e.g. a stereo system via cinch and headphones or active speakers via jack), with the same signal being present at both outputs. Please note, however, that these are not two separately controllable outputs - they are simply connected in parallel.

The Logitech Bluetooth Audio Receiver outputs the audio signal simultaneously on both outputs - i.e. both on the cinch output (red/white) and on the 3.5 mm jack output.