Way back when when I had my QH and TB, I didn’t always have enough time to ride them both so I would trail ride one and pony the other. These rides were: 1) on trails that were (obviously) wide enough for two horses and then some, 2) trails that the horses had been on themselves many times before, and 3) both were great trail horses in their own right.
The first time I ponied was the first time I ponied. Honestly, I didn’t even think of it. I got on my QH, took the TB’s lead rope and off we went. I had no troubles with it at all. I didn’t find it to be relaxing, though, as there was always some sort of obstacle to look out for (turn in the trail, joggers, bikers, other horses, people walking dogs, etc.), so I didn’t stick with it as a long-term solution.
With my current riding horses, a mustang and a four-year-old pony, I started ponying her before I backed her (so at three years old). I wanted to be able to do inside turns, outside turns, have the rope go over my mustang’s butt, under his tail, him stand and her move around him, her stand and him move around her, etc. These are useful things to practice and that’s where I’d start if I were you.
From the sounds of your descriptions, I’d be riding the QH and ponying the arab. But I’d sure as heck do lots of homework before going out. Going out with two antsy horses isn’t a good idea. Making a good trail horse is just as much hard work as making a good arena horse.
What I’d also do if I were you is make a tie spot (patience pole or some other) where you tie one horse while you ride the other. This isn’t hard-tying them to a solid object like a wall, but having a line hanging from above so that if they want to move around they are going to bend and step under themselves and get themselves all nice and flexible. And pretty soon they’ll learn to just stand there and chill out.
Also, ditto on the ground work and preparing your horse before you even go out on the trail. On a nervous or jiggy horse, I wouldn’t just hit the trail. I’d do ground work first. Then I’d ride in the arena. Then I’d use the trail as the cool-down place where life is nice and mellow and they don’t have to work (provided they are behaving). I’d then gradually move to an equal part of ground work, arena ride, trail ride. Then I’d do ground work, a little arena ride, more trail ride (starting to “work” out on the trails). Then I’d do ground work, trail ride. Then maybe just trail ride, depending on how my horse has progressed.
If you don’t have a horse who is already good on the trail, you need to train it just like you would train a horse for any other discipline.
A good trail horse is a MUST for me. I work on making them that way because I enjoy trail riding and think it is good for their brains and their bodies to get out of the arena (actually, most of my riding is on the trails and not in the arena!).