Is it possible to specify siteindex and urlset in one saitmap file?

Question

Search Engine Optimization

We had about several thousand pages on our site, about 300 thousand more have recently been added. I want to keep the old sitemap as it was and add some new ones. Is it possible to specify siteindex and urlset in the same saitmap file at the same time? those. in the old one, in addition to the existing url, add siteIndex, indicating an additional 6 new cards in it. Or basically you need to leave only index and store all url in separate maps?

<?xml version="1.0" encoding="UTF-8"?>

<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">

   <url>

      <loc>http://www.example.com/</loc>

      <lastmod>2005-01-01</lastmod>

      <changefreq>monthly</changefreq>

      <priority>0.8</priority>

   </url>

</urlset>

<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">

   <sitemap>

      <loc>http://www.example.com/sitemap1.xml.gz</loc>

      <lastmod>2004-10-01T18:23:17+00:00</lastmod>

   </sitemap>

   <sitemap>

      <loc>http://www.example.com/sitemap2.xml.gz</loc>

      <lastmod>2005-01-01</lastmod>

   </sitemap>

</sitemapindex>