Since I am actively monitoring updates in Google Search Console to make note of when Google Search organically index's the newest pages, I also happen to notice something interesting. When I first implemented a separate sitemap for AMP pages, I wasn't sure how Googles spider and related systems would like that idea. During that time, I did some research and couldn't find anyone that suggested to have separate sitemaps for AMP and regular HTML content. Since a "normal" sitemap and canonical references in the regular HTML pages should provide all that is needed for effective indexing of new AMP pages, there wouldn't seem to be any need to have two different sitemaps.
As I was thinking about how exactly Google handles AMP pages, including their indexing in the mobile first index and the use of the AMP cache system, I thought an experiment utilizing an AMP specific XML sitemap would be interesting.
Logging into Google Search Console and checking the sitemaps information shows that the amp version of the sitemap was read more recently then the standard sitemap. The dates showing are 12/2/18 for the AMP XML sitemaps last read and 11/27/18 for the standard XML sitemap.
In this posting, I wanted to at least note the dates of the last read. Maybe Google actually has a higher update frequency for AMP only sitemaps. That would make sense to me.