My alpha gelding can be a bit herd bound. Fortunately, it doesn’t manifest itself in royal hissy fits, but it is there.
When I first bought my horse trailer, he would first balk about getting on the trailer (just once or twice to let us know he was protesting the lack of a buddy on his trailer), then paw and throw himself against the wall in protest once he was on. He thought he should be on the big trailer with all his buddies. Honestly, I just ignored it and he got over it fairly quickly. I now trailer him1-3 times a week and I am able to load him myself. Once he’s up, you never now he’s there.
When I moved him to a new barn it manifested itself a bit. My previous barn was a large boarding facility. While they had a consistent feed and turnout schedule, in terms of time of day, the horses’ turnout schedules varied quite a bit depending on weather, tolerance, and who was at horse shows. He was sometimes the only horse in the barn or the only horse in turnout (he was relegated to private turnout because he is too alpha). He never fussed about it. His stall was also a bit isolated, so while he knew he had one neighbor on the other side of the wall, he couldn’t see other horses while in his stall.
Now he is in a smaller facility with a handful of horses. They all go out together and they all come in together. He has much more opportunity to socialize with horses, whether it is over the fence or in the barn. If he is last in or out, or needs to stay in, he fusses a bit. He also gave me a little herd-bound attitude under tack for a few rides. As enjoytheride said, as long as I gain his respect as the leader, he is fine.
I guess that is a long-winded way of saying, that based on my observations of my horse and several others in similar situations, my recommendations would be:
Earn his respect as his leader both on the ground and in the tack. My gelding is always better when his focus is on me and what I am asking him to do.
Try not to let him and your mare spend too much time together. Can you find a way so that they spend some time out of sight of each other for certain times of the day? Can you put them in separate fields or make it so they can’t see each other when they are in their stalls?
Can you practice separating them on the trailer? Maybe put him on the trailer and move him to another part of the property and let him work itself out? Maybe with a little ace or calming supplement at first and wean him off it? For my horse, once the trailer started moving he was fine, then at the shows, there was always something for him to look at, so he was pretty happy being on the trailer alone at the show. Will this work for your horse?
As much as I understand that horses are social animals, I also think that if they are to be competition animals, they need to learn to do things by themselves and I think people who keep their horses at home or places with a small number of horses need to be diligent in working with horses in small herds to prevent this behavior. In my experience, it is not so much that you need to find your horse a buddy, it is that he needs to figure out that he can function without a buddy. It is not an easy task.