First of all, please always creating new forum topic for your own questions instead of hijacking others'.
c) i dumped sql data from 1st server to 2nd and the from 2nd to first so that both have same data!
MySQL replication is better. But manually dumping is ok for testing.
d) setup unison for 2 way synchronization of /var/vmail/
May i know how you sync it on two servers?
A shared storage is better if possible. For example, NAS storage, NFS sharing, DRBD, etc.
On one of the servers roundcube displayed message could not connect to the database server and the second server displayed could not connect to the storage server
*) Roundcube cannot connect to the db server, you should check whether MySQL server is running. And the SQL username/password configured in Roundcube config file config/db.inc.php.
*) Roundcube cannot connect to storage server, please check managesieve service in Dovecot config file, and Roundcube plugin: plugins/managesieve/config.inc.php. e.g. listen port.