Can more than 1 horse be in a stall?

No . Since horses have a pecking order, disputes will arise from time to time. Therefore, it is not wise to put them together in such a small space ,such as a stall or very small pen. If one of them becomes aggressive, the other has no space to flee his assailant. Also, horses will deliberately box another up in a corner, even in a large pasture if it really has a dispute to settle.