A Sitemap index enables news publishers to manage multiple Sitemaps within the same account. You will need to create a second Sitemap if they have over 1000 URLs in one news Sitemap. Please keep in mind that we only crawl articles from the last two days, and that older articles/URLs will be rejected for Google News.
To learn more on when to update your Google News Sitemap, please visit this page. For instructions on how to create a Sitemaps index file, please visit this section of the Webmaster tools help center.