Before I got tired of synching and switched all my mail accounts to IMAP, I used Chronosync to keep my Mac Pro and MBP mail stores synched.
It worked well enough but sometimes (as when returning from a long trip) it was tedious. I like Chronosynch, but it's not free.
If you do the synching manually, don't forget to duplicate the mail preferences in Preferences (com.apple.mail.plist) because some of your account material is stored there.