I would agree with the other posters who say this isn't your call to make regarding the horses being kept separately. While I think most of us would agree that in an IDEAL world all horses would have at least one buddy to freely interact with there are all sorts of good, practical reasons that may mean this is not possible in individual situations. If the horses can see each other and perhaps visit over the fence a bit, they are still able to engage socially so are not in total isolation.
In my experience too in a large area with enough resources (food/water) spread out, horses will generally work it out. The current "herd" where I keep my horses consists of 1 QH gelding, 1 QH mare, 1 large pony gelding, and 2 medium ponies (1 mare, 1 gelding). They all get along very well in their ~15 acre pasture with either ample grass or more piles of hay than horses. However, if due to freezing rain or icy weather/footing we need to confine them in the paddock attached to pasture for their safety we end up putting the QHs in stalls in the barn. While everyone gets along very peacefully in the big area, once they are confined in a small area for a day or so the QHs get very pushy towards the ponies and will chase them out of the run-in shed, chase them away from the water trough, etc. - behaviors that we simply don't see when they all have much more space.