It is typically easier for me when the horses are at different levels as events around here usually run T/P on Fri/Sat and BN/N on Sat/Sun. So when I was competing two with one at Novice and the other at Training, I would have only minimal overlap on Saturday during an event. For example, a typical weekend would look like:
Friday: Training Dressage, Training Stadium
Saturday: Novice Dressage, Training XC, Novice XC
Sunday: Novice Stadium
When the Novice horse moved up to Training, that made it a little harder. And like others said, then it is all about good ground support, planning in advance, and keeping yourself to your schedule (for me, that meant staying focused on my own rides and not getting distracted by visiting with friends or watching others ride like I love to do). Sometimes it helped to have them in different divisions (OT and TH), sometimes not. Most secretaries are pretty good about recognizing you have multiple rides and trying to schedule you the best they can.
For me, the biggest issue was actually stabling. My main horse has MAJOR separation issues when traveling to events. I keep my small herd of horses at home, so all get turned out together and I don’t really have any other option on that. I ride each alone and separating at home while riding or doing other tasks has never been a problem. I can also take my main guy to events alone without issue or even pick up a friend along the way and trailer together without fuss. But if I take two horses from home, he turns into a totally different animal. I learned that the key was requesting my horses to be separated by at least one stall (usually asked for a close friend to be between them) if the stalls had full walls or more than one if they could see each other. P.S. It helps tremendously to make friends with the secretary!
Then, no stall guards, walked and grazed separately, and I would tack, mount, and leave the barn in opposite directions when riding so they never saw each other from the time we arrived until the time we loaded up to leave. It usually meant an hour or two of screaming and panic before my guy settled (the other horse always settled quicker) but eventually he would chill. It helped if I rode him first after unpacking to get his mind on other things. I ended up selling the other horse, but now have a young guy that is coming along and am starting to run into the same issues. Sigh.
Good luck! The key might be to show/bring three instead of just two… :lol: