N2 sync is there to force the ATS system to synchronize the engine core speeds, and not the fan (N1) speeds as it does normally. There isn't a practical use for this in normal operation, but if you select it and walk to the rear cabin you'll hear the distinctive sound of the fans being out of sync with each other.
You need to be in autopilot ALT hold, with a mach indication on the PFD tape rather than speed and the aircraft on the bug target mach.