Welcome to the Q&A Forum

As all 3 of us have said here, Pioneer, there is no issue with setting things up the way you are proposing. Can't make it any clearer than that. To answer your specific point - /sitemap and /sitemap.xml are categorically NOT seen as the same URL by search engines. They are absolutely considered two different pages. Your statement "...two items with the same url, but different file extensions..." is a non-sequitur. If the URLs have different file extensions, they are by definition NOT the same URL. The file extension (or lack thereof) is an integral part of the URL. Since 3 different people have given you the same answer and you still don't believe us, why not simply test for yourself? Implement the two files as above, then use Google Webmaster Tools to report your XML sitemap location, and confirm that it's finding and recognizing it correctly. Then use your browser to go to the URL of the regular sitemap and you'll see that it renders the html version of your sitemap map just fine. Paul