If you can afford another mouth to have as just a pasture puff, or even another rideable horse if you think you have time to manage 3, it's a good idea. I think it would also be a good idea to look into like a mini or a goat or something, if price is an issue. In the mean time, I think the progressive work would be best, if you can keep them calmly separated by a fence but within sight of eachother, and then gradually increase the distance and personally I would be physically working the one who reacts worse and letting the calmer one, if there is one, figure things out for itself. Best of luck! Buddy sour horses can be very difficult.