Also, regarding the Surface Duo, it's been done before, just not well.
https://www.cnet.com/reviews/kyocera...sprint-review/
https://www.cnet.com/reviews/zte-axon-m-review/
https://www.cnet.com/reviews/sony-tablet-p-review/
EDIT: Also forgot about the Nintendo DS line, though that's a completely different use case.