I'm not sure this is possible.
Having a backup MX is nice (mandatory from my point of view), but it's just a MX, not a full mail server : it won't allow your users to check their mails out of it (it won't know anything about the users anyway).
If you're concerned by your primary server going down, you should go the cluster way (RedHat Cluster is the supported way, but
Linux-HA + DRBD is reported to work too).