mmmmm. You can look at this from several angles I think. It might depend on how exclusively solitary the species is in its habits in the first place. Also how easy it is for the two members of a 'solitary' species kept either together, or in adjacent pens, to be able to avoid each other. Where is the boundary between it being a beneficial experience- or a stressful one....? (just asking the question..)
absolutely i agree it could flip the other way round and be stressful.
however - lets be smart here. we are talking about animal management and if there is one thing that clear to me, its that things arn't always clear!
animals, like people can be highly variable in behaviour and can have very different needs from other members of their species. the answer is to manage things on a case by case basis.
i'm not saying that maned wolf needs a friend. but i'm not going to be so extreme as place a blanket rule that "solitary" animals don't enjoy companionship in captivity.