If you have issues with Google indexing specific URLs on your site, Google says putting those URLs into new XML Sitemap files won’t help them get indexed. So for example, you have a Sitemap file with a 100 URLs but only 20 of those URLs are being indexed by Google, taking the other 80 URLs and adding them to an additional Sitemap file won’t get Google to index them.
John Mueller of Google said so on Twitter. He said it “wouldn’t change anything” to take this approach. Instead he said, you should work on making those pages of content awesome and then that would be a reason why Google would index them.