IMO it's one of those things that you couldn't recommend but might be possible with the right setup and individual fish-- for example, as mentioned, severums can vary in tankmates they'll tolerate; also, ime discus can vary widely in how timid or assertive they are.
I've seen (and done) some crazy things with discus. Pretty early in my discus years I kept them with all manner of crazy tankmates and had them be just fine, some of which I'd now say not to do, even though it worked at the time-- for whatever reason.
Based only on my own experience, while discus are naturally slow eaters that like to graze and pick, I've had them adapt to a busy tank and compete just fine for food, IF-- and it's a big IF-- they still feel comfortable in the tank, despite whatever unconventional tankmates... among the fish I successfully kept with discus were mesonauta and red tailed chalceus (these weren't the craziest I tried), never tried severums and discus, though. (For the record, over time I became more conventional as a discus keeper, mainly for aesthetic reasons.)
The potential rub with doing something unconventional with discus is if they're stressed or uncomfortable they can stop eating or do the turn dark and hide in a corner thing. Not a problem I had back in the day, but not something you want, either.
