I don't have dual monitors to test ...
ScreenSavers were popular before Deskscapes but made in a lot lower resolutions
most would not work for you now with that set up .
Not sure what your " Custom ScreenSaver " res is.
I have made a lot of ScreenSavers a few lately , try a few of my more recent ScreenSavers and see if they work any better
My ScreenSaver link : http://azdude.wincustomize.com/skins.aspx?libid=40
My recent SS are made at HD 1080p res. . none here are made UHD res.
Do the pre installed Windows SS's work with your set up ?