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

luckyduck.networks

This plugin, once activated, generates a sitemap.xml on-the-fly for every site in a multisite network. The sitemap of every site in the multisite only contains location entries which are related to that particular site. The individual sitemaps don’t contain traces to other pages in the same network.
<h3>License</h3>
This plugin is released under the conditions of the GPL license. You’re free to use it free of charge on your personal or commercial blog.

Simple Multisite Sitemaps

This plugin, once activated, generates a sitemap.xml on-the-fly for every site in a multisite network.

IsSiY (Include second Sitemap into Yoast) WordPress SEO Extension vs Simple Multisite Sitemaps