[Brian]

pjb, the General pane of Preferences still has all the controls that this thread refers to; you can still switch between different databases on your desktop if you like.

I would not recommend that you do this while syncing is turned on, though - things will very quickly get very confusing for you and OmniFocus both.

Pick one database you want to sync, and before you load up one of the other databases, set your sync preferences to "Sync with: nothing".