If I recall correctly, you're using the type of headphones that have a base transmitter, sending out signals to those headphones, rather than Bluetooth headphones (where there's a transmitter built into the device that the audio signals are coming from). It makes a difference!
With Bluetooth headphones, the sending device and the headphones are 'paired', so that the signals are encrypted between the two devices. That means that only the person using those headphones can hear the sound from the relevant device.
With (what I believe) to be your type of headphones though, the transmitter unit (which plugs into a 3.5mm audio output socket on the device) works in the same way that a transmitter sending out signals from Radio 2 does. Just as anyone within the relevant reception area can tune into Radio 2, anyone who's got a 'radio' tuned to the right frequency can hear the output from your little base transmitter unit. Both of your pairs of headphones are, in effect, 'radios' which are pre-tuned to the frequency used by the base unit. So both of them will receive signals from it.