I have been searching around using Google but I can't find an answer to this question.
A robots.txt file can contain the following line:
Sitemap: http://www.mysite.com/sitemapindex.xml
but is it possible to specify MULTIPLE sitemap index files in the robots.txt and have the search engines recognize that and crawl ALL of the sitemaps referenced in each sitemap index file? For example, will this work:
Sitemap: http://www.mysite.com/sitemapindex1.xml
It is possible to write them, but it is up to the search engine to know what to do with it. I suspect many search engines will either "keep digesting" more and more tokens, or alternatively, take the last sitemap they find as the real one.
I propose that the question be "if I want __ search engine to index my site, would I be able to define multiple sitemaps?"
Yes this is possible, see in the sitemap.org site. http://www.sitemaps.org/protocol.php#submit_robots is the link. They clearly states that it can be done! Google, Yahoo and /or Bing, all are members of sitemaps.org. So you can be assured that your sitemap entries will be properly read by the search engine bots. BUT as John said above, better submit them via webmaster tools also, for added assurance.