Multiple XML sitemaps can be used to help detect how the indexing of the site is progressing or if there are any problems. For example if you have 20,000 pages in the site and only 15,000 are indexed it’s hard to find out which 5,000 pages are not indexed by the search engine.
Google Webmaster Tools feeds back on this information saying how many pages have been indexed from each XML sitemap, best of all you can submit multiple sitemaps to the search engines.
Breaking the sitemap down can pinpoint which sections of your site are not getting indexed. So lets break that 20,000 pages down.
So we can see that the main problem is with the products section not getting indexed with only 44% of pages indexed. This now means you can start to find out the reasons why your product pages are not being indexed.
That then opens a new can of worms as it might be due to duplicate content, poor coding, internal linking, server issues etc. The good point is that you now know what area is the problem.
To link the XML sitemaps together you should have one sitemap called sitemapindex.xml that links to the other sitemaps using the xml protocol. Submit all the sitemaps to Google Webmaster Tools and list all the sitemaps in the robots.txt. E.g.
So if you think your site has an indexing problem then split your site down logically into different XML sitemaps. It’s not going to solve your problem but will point you in the right direction.