Rays will kill discus if they can catch them off guard and land on top of them while they are horizontal, and they sleep horizontally a lot. Also discus I personally have owned have a bad wrap for attacking eyes when they fight. Considering the rays eyes are easy large targets you may come out in the morning to half dead blind rays. Not a great combo of fish unless it’s a super large tank with large areas the discus can get into but the rays can’t. It’s a 50/50 I’d steer clear.
