Thanks, Pieter, for clarifying this issue for me. See, the two sites offer different services, even though they both target the same field (South American anthopological linguistics). One is a general repository of information on indigenous South American languages (http://www.etnolinguistica.org), including dissertations, journal articles, news articles, a professional directory, etc. The other (http://biblio.etnolinguistica.org) is a digital library of hard-to-find books and articles on South American languages. I created them first with Geocities, then moved to Googlepages, and then Wikidot (of course, by far the best solution, which unfortunately wasn't available when I first created the sites).
Now that I'm more familiar with Wikidot I realize that I should have unified both sites back then, so that users wouldn't have to join twice, tags and search would be unified, etc. I could keep the sites' separate look and feel with categories, etc. And that's what I want to do now, before the sites get even bigger and before I launch a campaign to have all the members of our Yahoo mailing list (numbering around 550) join the site and slowly phase out the Yahoo Group. The library site has around two hundred pages, which I'll have to migrate one by one (the most painful part, I guess, are the pdf files, which I'll have to download to my computer and then upload again). But it has to be done, and the result will be a more powerful and efficient tool for our community.
One thing that would make everyting much easier would be the possibility of mapping the subdomain biblio.etnolinguistica.org into a category of the main site, in such a way that, say, a book located at biblio.etnolinguistica.org/book1 could be automatically found at etnolinguistica.org/biblio:book1. It would work with folders (etnolinguistica.org/biblio/book1), but it doesn't seem to work with categories. Any ideas, Wikidotters?