The master is the one that connects to the rest of your gear, so in this case the one off stage would be the master. It doesn't really matter about the terminology, though. Whichever unit that doesn't have another GT attached to its MIDI Output will be considered the master.
Synchronization works well in both directions, we've had bands using this feature with 2 (sometimes more) GTs for at least 8 years now.
The pinout is just a normal one-to-one wiring, just make sure that all pins are wired through. To have bidirectional communication, all 5 pins are needed. A GT can't phantom power another GT, so you would need a phantom power box or cable to inject the phantom power from the master to the slave.