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

Mike Koepke

Denis de Bernardy

The XML Sitemaps plugin for WordPress will automatically generate XML Sitemaps for your site and notify search engines when they’re updated.
Contrary to other plugins that generate sitemap files, this one will add a rewrite rule and store your cached sitemaps in the wp-content/sitemaps folder.
The XML Sitemaps plugin automatically assigns the rate of updates and the weight based on statistics collected on your site.
Pings occur automatically, on an hourly basis, if the sitemap file is updated.
Lastly, and contrary to the zillions of plugins that try to do the same as this one, this plugin will use the WP internals to determine the number of blog, category and tag pages on your site. This means it’ll play well with the likes of custom query string or <a href="http://www.semiologic.com/software/sem-seo/" rel="nofollow ugc">Semiologic SEO</a>.
<h4>Configuration</h4>
Starting with version 2.0 there is now the ability to control the inclusion or exclusion of date archives (years and months), author, category and tags pages in your sitemap.
Additionally specific pages can also be excluded by entering their page IDs.
Go to Settings->XML Sitemaps to update settings.
<h4>Help Me!</h4>
The <a href="https://wordpress.org/support/plugin/xml-sitemaps" rel="ugc">Plugin’s Forum</a> is the best place to report issues.

XML Sitemaps

Works well and the code behind it is simple.
I would like to contribute to this plugin. Please contact me about how I can get you my update that adds AMP pages.



Good and simple.

Automatically generates XML Sitemaps for your site and notifies search engines when they're updated.

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