Re : merge 2 tikiwiki content
If the first one is "mainly" wiki pages, perhaps you can use the export/import feature in admin->wiki. There are some bugs (if you have pagename with special character ... ) so save your data base before. You can use also the mail-in feature to copy the pages from the first db to the second db- but an action for each page.
But so far I know you will loose page category, author ..... It is only about content...
It will be a nightmare
From my experience I don't know an easy way
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