Merging MediaWiki contents together
MediaWiki has import and export features, that you can use for moving the contents. It does merge histories of existing pages (see import link).
However, it does not handle conlicts of two different pages with the same name, so you'll have to handle moving the pages by yourself. As far as I can see in description of MediaWiki page table, you might be able to find and rename duplicate pages directly in SQL, but pywikipedia should work too (but you'll need to delete redirects that will result from "correct" page moves, otherwise they'll mess up with imported pages from the other wiki).