The library is tied to your user folder so the easiest way to have multiple libraries is to have multiple users.
That said, it's conceptually simple to do this, since Finder is scriptable and the filesystem is, in general, given to manipulation.
The data are in ~/Library/Application Support/Delicious Library/ and if you have a bunch of folders and used AppleScript (or whatever) to rename one at a time you'd basically have a library manager.
You may also want to swap out the plist, or you'd lose your preferences (which may or may not be a big deal) but you definitely want to swap out the Spotlight metadata folder or you'd have to wait for Spotlight to reindex your library every time you swapped, which would take forever.