how to start a replication master-master correctly

Hi all, I currently have one dovecot 2.2.15 working and I would like to add a second master server. If I configure the second server and enable the dsync in both servers, the sync will copy everything from A to B ? I mean, B server won't remove messages in A as it doesn't have anything in its users mailbox?

The second master server will be a cold server just for contingency.

Thanks