In my CS60, such a mod was done.
The Gate signals were simply connected together in pairs (with a switch), apparently forming some weird wired-OR.
In a 3rd switch position, the Gates were connected in two quadruples, obviously aiming at a two-voice, 4-note stack.
First thing I did was getting rid of this modification.
It _sort_of_ worked, but not really perfectly. Probably becaus eof the comlex voice allocation in the CS-Synths.
The right way to do it, IMO, would be like this:
In 4-Voice mode, use the output pin for the 5th Gate as an input pin with the right logic level (look up CS-50 and CS-60 schemos for details), and connect Voice board 5's CV to Voice board 1's CV, 6 to 2, and so on.
Disclaimer: I have *NOT* done this myself, I do not really recommend it (otherwise I would have done it on my CS-60), and apart from this mail, I won't be of much help if you want to do this in yout precious instrument.