About fuel, the custom datarefs are tied with the 3 fuel cursors and the Apply Settings button. As for the 2 karts, and any other items of the popups, we have no datarefs and/or commands to synch them. In fact, even stock fuel datarefs could be deleted, as they are in my last file. Only payload (sim/flightmodel/weight/m_fixed = 0) and CG (sim/flightmodel/misc/cgz_ref_to_default = 0) are synch frome master to slave.
You can enable the fuel datrefs, open your fuel and payload tabs (X Plane), and move the cursors: payload and CG are OK (as joystick settings), fuel will flicker for the slave between the values of the master and its own values.
Remaining problem with fuel: the custom dataref of fuel flow does not match precisely (despite stock dataref is OK), After a long flight, there is a little difference with the predictions (probably a fast PC vs a slower one).
sim/joystick/joystick_XXXXXX: Not mandatory. I generally fly as master, my settings are saved with xassign, so i am sure that both pilots have the recommended settings by the devs.
Claude.