Do horses need separate stalls? The answer to this question depends on the horse, and also on the stall. In a barn with average sized stalls, average sized horses absolutely need to be kept in separate stalls. Most stalls are 12X12 feet, with potentially a few feet variation. This is a fairly small confinement space for a horse, and to share the stall with another horse instead of having separate stalls for each usually leads to one horse being injured by the other. (and in a confined space, unlike a pasture, the victim horse is unable to escape from kick-range in a stall)
Sometimes horses do not need separate stalls, but these are unusual circumstances such as: An extra, extra large stall (example, a large indoor round pen might be able to house two or three horses) and/or small horses who have been companions for years and who absolutely will not fight. Each horse needs space to lay down on their side and stretch out, so most horses need to be put into a separate stall to ensure they remain safe and able to rest comfortably.
So yes, with only the most unusual exceptions, horses need separate stalls.