Direct groups of users to pairs of backend mailstores?

[Murray Trainer]

Hi All, 

We are using Dovecot in a Director setup with two director proxies,
six backend mailstores, mailbox storage in NFS, and user accounts
stored in LDAP.   Currently we have five NFS filesystems on EMC VNX
storage which are all mounted on each of the six backend
mailstores.   At the moment all users could be directed to any of
the six mailstores.  We have a group of users with very large
mailboxes we want to put on separate cheaper Backblaze NFS storage. 
Because this will be less reliable than our VNX storage we want to be
able to direct this group of users to a new pair of backend mailstores
that only have the Backblaze NFS mounts.   This would isolate the
rest of the users isolation from problems with the Backblaze NFS
servers.  I have read the Dovecot documentation which indicates it is
possible to direct users always to a single backend server.    Can
Dovecot direct users to a pair or more of backend mailstores - ie a
subset or another set of mailstores.    If not it would be a great
enhancement which would allow more scalability, flexibility and
redundancy.

Thanks 

Murray