Would someone running W10 pro on a 2 head rig try a (slideshow) theme
(Official MS downloaded theme. 2 tried)
What I am seeing

on changing to a new theme):
1st (in sequence) slide on Monitor 2 (extended)
2nd slide on Main
Monitor 2 never changes slide
Main monitor changes (waiting for the sequence to repeat, to see if slide 1 ever shows there)
W7 showed the same slide (in sequence) on both monitors.