You're right, this has been mentioned before.
Captain Sim doesn't support multiplayer operations at this moment, so I'm afraid it just won't work.
The ability to share the cockpit needs to be built in by the developer (or rather, not built out) it is indeed, as you say, the custom gauges and programming that make in incompatible.
I think there are already topics on this in just about any CS it'd be nice topic, for every aircraft.

maybe, just maybe, somewhere in the future, who knows...