Replication is in the current dovecot version but will go away in 2.4.
The doveadm sync feature is staying. So with some work you can set it up what you are requesting.
I used to use replication and now I'm thinking about using sync but have not implemented it. The following are thoughs on it.
There are some points to be addressed that are outside dovecot. I think you'd have to make sure that your sync happened frequently enough that you could live with losing the emails that arrives bewteen syncs for example.
I have been thinking of writing a hacky delivery script that passes the email on to dovecot-lda, then runs doveadm sync, and only returns success after the sync is done (or after a timeout). No idea what problems I will run into, but the idea is to never accept an email until it's guaranteed replicated.
That would tend to lead to a requirement to sync more frequently and reduce risk of email loss. But then you'd need to avoid more than one sync being active simultaneously (that is my assumption that this would not work, but I don't know if it is a real problem).
Doesn't the -l option protect against simultaneous syncs? Just based on reading the doveadm-sync man page. (I guess it could cause a problem if you start sync processes more quickly than they can finish.)
NB I'm just running a one-person email server so don't take my ideas too seriously :)
-- James