The compressor stereo switch links to the "A" (compressor side chain) board, via the A16 edge connector. It effects the compressor and the expander sidechains, but does not effect the peak limiter sidechain, which remains independant.
From looking at the service manual, and the owners manual it looks as though it shouldn't even have the switch there on the F769X-R, the manual says that the switch is for stereo units only, so I don't know what it is on this unit for. They suggest setting each channel up seperately, and then throwing the switch.
Oddly enough, my unit does have a switch, while in the manual the photo of the unit has a blank plug in the switch hole. The photo of the F760X-RS stereo unit does have the switch. It has got me thinking now that I had better pull my unit apart and see what that switch is connected to, perhaps some studio tech in the distant past has used the hole to put a switch into for some other function entirely...
So getting back to your point, if you are lucky enough to have 2 of them then it should be a simple matter to put a stereo linking connector onto the unit if you don't mind drilling the case, but I would be suspect of that switch if your units have them, because mono units look like there is not supposed to be a switch there at all, just a blanking plug. But linking the 2 sidechain amps via A16 should get you going, don't forget the instruction to set up both units seperately before linking.