If you have opened multiple databases in DEVONthink Pro 2.0 public beta you can easily switch between them using the new sidebar. But if you want to move documents between databases this it not yet comfortably possible. The sidebar, which will allow you just this, is not fully functional in public beta 1 (but it will become functional in one of the next public betas).
However, you can easily open multiple windows in DEVONthink Pro and use each of them to view the groups and documents of one database. Use drag-and-drop to move selected items from one window to the other; when you move an item from one database to another it will be copied, not moved, similar to when you drag a file from one volume to another in the Finder.
Alternatively you can use the Groups panel (Tools > Show Groups) to access all groups of all open databases, e.g. for drag-and-drop.