Have read a few articles on this but not seen it in real life. from memory some of the issues were :
Type of ray - biggies upset and eat discus
Rays like to hide in the substrate a lot
Feeding - rays like a different diet - lots of seafood i recall
Temperature - think rays don't like the discus temps
Sorry can't be any more help, but have not done it or seen it first hand.
The fish store by my house (brooklyn, ny) has a huge discus tank. Inside those tanks are a couple of pretty big rays. They told me they seem to get along fine. (hystrbix rays or something like that). They both need warm water and about the same water parameter the guy at the store told me. They are usually smart and know their stuff. Hope that helps.
Sounds like a risky venture to me. Maybe in the future when I have more tanks and can move guys around tank to tank with "trials". Thanks for the posts.