A bit late to the party, but the commands for this are:

/script QuestSessionManager:StartSession() Initiate sync (run on one toon)
/script QuestSessionManager.CheckStartDialog:Confirm() Confirm starting a sync session (same toon as above)
/script QuestSessionManager.StartDialog:Confirm() Confirm joining a session (run on the other toons in party)
/script QuestSessionManager:StopSession() Run on the first toon to stop the sync session
/script QuestSessionManager.CheckStopDialog:Confirm() Run on the first toon to confirm stopping

I guess it's not too hard to set it up quickly with broadcast enabled, especially as it's only once per session. But you could bundle these into some macros if that's your preference.