If the user wants to listen to matrix two ports, that easy. Take the blue pair from the RJ45 CAT cable of each port and send them to pins 2 and 3 of two XLR male connectors and put them in a stereo headphone amplifier. Float the ground.

But if they want to listen to a panel’s headset PLUS an extra 4wire port from the matrix, that’s s bit more interesting:
There are several options a) order a V-series panel with a 5 pin XLR headset - wire the extra 4wire port locally into the back of the panel Aux Audio DB25 connector (pins 25, 12) and set the panels Audio DSP mixer as per the attached .. except the EXT1 connection is heard via the HS1R (headset 1 right ear) and the main intercom is heard via HS1L (left ear)
b) Using a AES enabled panel you can send 2 audio chnls from the matrix to the panel.Again the panel Audio DPS mixer is set up so that matrix 1 is on HS1L and Matrix 2 is on HS1R Again this uses a 5 pin XLR headset panel 
c) using a standard 4 pin XLR headset panel On the back of the v-panel, there are two DB-25 connectors. You’ll attach to the male DB-25 (closest to the Ethercon Port RJ45 connector).Attached is the pinout and the screen shot from the v-panel mixer in the ECS/EHX software panel tab.Wire the headset with:Female XLR   to DB-25 femalePin 1                      Pin 14Pin 2                      Pin 1Pin 3                      Pin 15 & 16Pin 4                      Pin 2                      Pin 5                      Pin 3
Pins 25 and 12 of the DB 25 attach to the blue pair from the RJ45 from the 2nd port that you want to listen to (RJ45 pins 4&5)
This will give the user the matrix (panel) audio in their left ear, controlled by the MAIN Volume Knob on the V-panel AND the right ear will hear the extra port, controlled by the AUX Volume knob.
See attached drawings