Ponying isn’t a good training tool for separation anxiety, because it is all about the horse relying on the other horse for cues on what to do… but it is a good training tool to have in your toolbox especially when you only have time to work the one, or you want to expose a green horse to things off of the back of a more relaxed/experienced leader.
Ditto Foxglove - the first thing to do, is teach this horse to be respectful of your space on the ground, and understand verbal & visual cues like “back up” and stopping immediately when you stop. They also should know all verbal cues for W/T/C. When you confirm these cues on the ground, it easily translates to the saddle if you’ve trained them well.
Start small in a closed area, and keep the horse’s head level with your stirrups/knee to start with - NO slack on the rope. I primarily prefer to pony off of my right side, but you need to teach them to pony off either direction - there are times on the trail where you will need to pony off of a specific side and that’s not the time for the horse to say ‘wait a minute, we’ve never done this before’.
A good general rule is to use a rope halter for the one being ponied, and don’t let there be much slack in the rope. You don’t want the rope to end up under your leg, or around your foot. Keep the horse to your foot/knee and work on the horse stopping when your horse stops. Do figure eights, halting in the middle of each change of direction - until the horse is actively watching you/your riding horse. They should always be looking to what your horse is doing for their cue, IMHO.
I don’t do much ponying at speed works, but I do pony trot sets.
If you find your ponied horse is getting ahead of you, make sure you forbid that. Bring your horse to a halt and make the ponied horse back up until he is behind your leg. Then try again. Every time they get ahead, do this - until they learn they can’t forge ahead.
With a more experienced ponied horse, I tend to let them follow behind rather than be by my knee. There are some parts of where I condition that are not wide enough for two horses abreast – but the more slack you allow in the line, the more freedom the horse has to get ahead or away from you… so it’s best to keep them on a tighter leash so to speak when starting out.
Make sure the horse you’re riding has a good sense of humor and is rope broke. You will be untangling yourself eventually - part of the learning process, especially with the green ADHD babies.