And now, from a single host, you can submit Sitemaps for both sites without any errors.
sitemap-example.xml contains URLs from www.example.com and
sitemap-mysite.xml contains URLs from mysite.google.com but both now reside on www.example.com:
We've also added more information on
handling cross-submits in our
Webmaster Help Center.
For those of you wondering how this affects the other search engines that support the Sitemap Protocol, rest assured that we're talking to them about how to make cross-submissions work seamlessly across all of them. Until then, this specific solution will work only for users of Google Webmaster Tools.