Your headphones come with a 3.5mm sharing splitter. This allows two headphones (or headphones/earphones) to connect to one audio source, and enables two listeners to listen to the same audio at the same time.
To use the sharing splitter:
Connect the 3.5mm connector to your smartphone or other audio source with a 3.5mm connector.
Connect your UE 4000 to one of the ports on top of the sharing splitter.
Connect the second headset/earphone/headphone to the other port.
NOTE: The UE 4000 can also be used without the sharing splitter.