You may already know that you can create smart groups in the sidebar of DEVONthink Pro (Office) that collect documents from all open databases. If not: click the action menu button at the bottom of the sidebar and choose New Smart Group.
What you might not already know is that you can also search the contents of a smart group (which is itself just a saved search). Select the smart group in the sidebar, then click the magnifying glass icon in the window’s search field and select In Selection. Your search will now be restricted to whatever is currently gathered by the smart group.