They will likely become quite attached and bonded. That doesn't necessarily mean they cannot be separated out of sight from one another.
Some horses don't seem to mind having their buddy taken away, they holler a bit, and then calm right down.
Others pitch a major fit and get really worked up, sometimes to the point that they endanger themselves or you.
You'll just have to see what happens, and react accordingly.
IMO, if you have a horse that panics when alone, the heat of the summer is NOT the time to deal with the issue. A severely buddy-sour horse can quickly become overheated and stressed, even if confined to a stall or small area.
