> I'd like to combine two databases adding their galleries together. Is this possible?
No, I'm afraid not. Theoratically, if you could manipulate the databases directly (using Access or another tool) and the thumbnails have the same ThumbnailID values, it might be possible - but not from within ThumbsPlus.
If it's important enough to justify the work, you could add a specific keyword to the XMP (IPTC) keywords of the files in a gallery. For instance, if your gallery holds landscape pictures, you give all those images the keyword 'Landscape' (using Image | Batch Edit Metadata). Then you open the database to which you want to move the gallery. You tell ThumbsPlus to copy XMP-keywords into the database when it thumbnails images (Options | Preferences | Keywords). Then you (re)thumbnail the images and you perform a database search for all images with the keyword 'Landscape'. Add all the found images to a new gallery named Landscape and voila, you have moved your gallery to the other database :)