E. Marten

You need to use WordPress SEO by Yoast (Props to him!)
Adds “More Sitemaps” to your WordPress SEO Settings. If you are using a second WordPress-Installation in a subdirectory, you can now easily add the second Sitemap into the main one.
If you are using a second WordPress-installation in a subdirectory and want it to be fully integrated it into your main site to prevend struggeling with two or more Sitemaps in your Webmastertools, 
you can easily integrate more sitemaps into your main one. (sitemap_index.xml) 
The Plugin adds your second and third (aso) sitemap into the main sitemap and tooks care about the modification date automatically!
Thanks to Joost de Valk!

IsSiY (Include second Sitemap into Yoast) WordPress SEO Extension

Adds "More Sitemaps" to your Wordpress SEO Settings. Include differen sitemaps to your sitemap_index.xml

Christopher Churchill

If you have any feature requests I’d be more than happy to implement them just open up a thread on github please:
https://github.com/vimes1984/video_sitemap/issues
All errors to be tracked via github please: https://github.com/vimes1984/video_sitemap
This is a little plugin to automatically scan a sites post content and metavalues for youtube liniks and automatically generate a video_sitemap.xml.

Youtube Video Sitemap generator

Scan your site for youtube links in both post content and meta tags and create a xml video sitemap file on the fly.

IsSiY (Include second Sitemap into Yoast) WordPress SEO Extension vs Youtube Video Sitemap generator