Hi msquare,
According to Identity on a blog post about multiple sitemaps:
It is still just one URL, but the engines prefer to not have URLs duplicated across sitemaps.
In this way, you are sending a lower quality signal about your sitemaps. Certainly this can happen, like any duplication, but if it is rampant, hundreds-thousands of URLs duplicated across multiple sitemaps might lead the engines to believe that your are trying to manipulate them in some way and I wouldn't be surprised if they trusted the sitemaps less, which may defeat the purpose of having them in the first place.
I would argue the same. If you have two sitemaps with lots of similar content then it can be seen as inaccurate which is contrary to the point of the sitemap in the first place.
Have a read of that article too that I linked above, because there's a good introduction to multiple sitemaps and indexes, so that you can avoid as many problems as you can!