Multiple sitemaps
After you have setup your license and installed the additional Sitemap expansion you can now manage multiple Sitemaps from the Strapi admin panel. If you haven't do so yet, please follow the instructions here.
Configuration
Navigate to the Sitemap settings page in your Strapi application. In your local development env you can find that on the following URL http://localhost:1337/admin/plugins/webtools/sitemap.
Here you are now able to see the different sitemaps that you have created. By default you will have a single sitemap already made with the name 'default'.

Access
Each sitemap can be accessed individually through the REST api. The URL to access the sitemap will be as follows:
http://localhost:1337/api/sitemap/[sitemap-name].xml
In the URL as shown here, you'll have to replace [sitemap-name] with the actual name of the sitemap that you want to access.