In general, backup to an image isn't really a workaround: it's a backup. You're doing something quite different than backing up, which is why it doesn't work well for you.
What I'd suggest is using an IMAP mail server. That's the best way to keep mail synchronized between two Macs.
Trying to do this by syncing the data folders with an external program is asking for trouble as storage formats change and become more complex (or move)...
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--Dave Nanian
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