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

MikeIrvine

This is special plugin which helps your WordPress site to easily create sitewise XML sitemap and ping Google, Bing, Yahoo and Ask.com to index your WordPress site or blog. This plugin generates XML sitemap following sitemap.org Sitemap protocol to help search engines like Google, Yahoo, MSN to better understand and index your blog. The Sitemap protocol enables you to provide details about your pages to search engines, and we encourage its use.
The XML sitemap created using this plugin satisfies completely with Sitemap.org protocol which says
The Sitemap must:
*Begin with an opening tag and end with a closing tag. 
*Specify the namespace (protocol standard) within the tag. 
*Include a entry for each URL, as a parent XML tag. 
*Include a child entry for each parent tag. 
All other tags are optional. Support for these optional tags may vary among search engines. Refer to each search engine’s documentation for details. 
Also, all URLs in a Sitemap must be from a single host, such as www.example.com or store.example.com.
With such a sitemap, it’s much easier for the crawlers to see the complete structure of your site and retrieve it more efficiently. The plugin supports all kinds of WordPress generated pages as well as custom URLs. Additionally it notifies all major search engines every time you create a post about the new content.
It will do exactly what it’s supposed to do – providing a complete XML sitemap for Google, Yahoo, MSN, ASK and other search engines. It will not break your site, slow it down or annoy you. Guaranteed!
<blockquote>
If you like the plugin, feel free to <a href="https://www.ijunkey.com/rate" rel="nofollow ugc">Rate it 5 star</a> or <a href="https://ijunkey.com/buymecoffee" rel="nofollow ugc">Buy me a coffee</a>.
</blockquote>
Thanks a lot! 🙂
<h3>License</h3>
Good news, this plugin is free for everyone! Since it’s released under the GPL, you can use it free of charge on your personal or commercial blog.
<h3>Translations</h3>
The plugin comes with various translations, please refer to the <a href="https://codex.wordpress.org/Installing_WordPress_in_Your_Language" title="Installing WordPress in Your Language" rel="nofollow ugc">WordPress Codex</a> for more information about activating the translation. If you want to help to translate the plugin to your language, please have a look at the sitemap.pot file which contains all definitions and may be used with a <a href="http://www.gnu.org/software/gettext/" rel="nofollow ugc">gettext</a>.

Google Sitemaps XML

This is special plugin which will generate a special XML sitemap to help Google, Yahoo, Ask and other search engines to ndex your site or blog.

IsSiY (Include second Sitemap into Yoast) WordPress SEO Extension vs Google Sitemaps XML