The design of the Sync plugin contemplates that the database may be open on more than one Mac, simultaneously.
But this is still a beta, currently public beta 3. Check the Blog occasionally for notice of a new beta, if another is released.
As this is a beta, please note the warnings about using it for your working databases, ESPECIALLY the advice to retain backups of the database before a Sync attempt.
Sync is not instant. It operates on the defined schedule or you can manually Sync when you want to. Each machine in a Sync ring (so to speak) has its own schedule.