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You should be able to use the sync facility to sync to disk in a shared location (such as a directory in /Users/Shared) that all the users can read and write. You'll want to click the sync button before and after switching to make sure you've got everything. If this isn't enough detail, I can flesh it out a bit more.

I use fast user switching so that I can have different OmniFocus databases, not the same one :-)