Lets see if I can explain this...use one wire and run it from deraileur #1 through some housing to a cable stop. Now make a loop and run the wire through another stop and some housing back to deraileur #2. Now use a cantilever brake saddle and connect it to the main shifter wire. Hook the loop over the brake saddle. Now when you shift, the wire from the shifter is pulling the saddle which in turn pulls the wire that connects to both deraileurs.
Does that make ANY sense? PM me or call 831-521-3801 and I'll try to clarify.