I'm planning to make a WordPress site and include, also, a wiki, using MediaWiki. I know how to generate WP and MW sitemaps. Anyway, I think I need to merge the two sitemaps in some way. Am I right? How can I do this?
You don't need to merge the sitemaps, you can list multiple sitemaps your robots.txt file:
You can also submit multiple sitemaps to Google via Google Webmaster Tools.