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If you only need OmniFocus open on one machine at a time, you actually only need to purchase one license. A single user may have any of our apps installed on two machines with the same license. Just quit it on one machine before launching on the other.

While .Mac syncing sounds good in theory, in practice it hasn't proved sufficiently reliable. What I'd recommend (and I know some folks, even some OmniFolk are doing) is to put the database on a USB thumbdrive.