So I played with this a bit and can make s suggestion. On the GS700 there is a 3rd monitor output that is intended for recording but has other functions as well.
I set up a system with the dual camera set up, then set the 3nd monitor to Record Mode. Teh output on the 3rd display was the composited image of the dual cameras. I think that will provide you the output you are looking for, possibly connect that 3rd output to a different monitor input.