It is very easy to add multiple sitemaps to the robots.txt file of your website; simply list them one after the other using the Sitemap directive.
Here is an example:
User-agent: *
Sitemap: https:// www.example.com/sitemap1.xml
Sitemap: https:// www.example.com/sitemap2.xml
Sitemap: https:// www.example.com/sitemap-blog.xml
In the above example, put your domain name and sitemap filename.