Multiple Installs / Sitemap

Okay, here’s the scenario

I have a WP install

domain.com/

With the sitemap

domain.com/sitemap_index.xml

Then because I wanted to run a copy of the site with more plugins and didn’t want it to burden the whole site I installed another instance of WordPress at

domain.com/learn/

If I install Yoast on this install I end up with

domain.com/learn/sitemap_index.xml

Is there a way I can

a) Reference the sub folder sitemap in the main sitemap

b) Change the name of the sub folder sitemap to course_index.xml?

Or is the best method just to submit the second sitemap to Google Search Console and be done with it?

Thanks