Well, I want the character to make a very narrow, haphazard escape. That's not a problem. The problem is making it believable.
There are only two horses, kept in a depression in the wild. How steep are the sides? You need to understand that a horse can climb as well as a mountain goat, without a rider. Depressions work for hiding a horse but doesn't help at all for keeping one.
Probably there are only going to be one or two ways easily out of the depression. Trust me that the horses will figure out the easy way and that's the way they will head. Most likely path the horse will take is the same way it went in. But that's not a problem.
Two characters flee there, take the hobbles off the horses, the enemy shows up, cuts one of them down. Assuming primitive technology, the hobbles would be rawhide. Basically a figure-eight shape on the front legs (there are also three-legged hobbles, but you wouldn't use them in this case). Hobbles limit motion, but depending upon the horse, they can run at a full gallop in them. Especially when scared. Solution to the hobbles from the rider's point of view is just a slice between the front legs. However, the chances of getting this done on a freaked horse is questionable. Those front legs are going to be thrashing all over the place. Personally I'd ignore the hobbles, which are probably going to break pretty soon anyway. They're not designed for this sort of abuse.
Any rider experienced enough to stay on this horse would not need to dismount to get rid of the hobbles. Remember that primitive horses tended not to be that tall. There's no way I'd stop to deal with this against unmounted pursuers.
The other climbs up on the freaked-out horse (his own, not a war-horse) No. You would not climb up. Safest way is to vault onto the horse as the horse runs by you. A freaked out horse wants to run. They believe in flight, and will only fight if there is no way out. But assuming the horse is staying in place (WHY???), approach would be to vault onto the horse.
, and clings on for dear life and tries to steer the horse away from the enemies, while they in turn try to cut off his escape. The horse will run over the enemies, so they're not much of a problem. Probably would not worry about steering at the moment.
In battle, this scenario happened frequently. Warrior's horse would be cut down, and a person standing on the battlefield is chum. Best approach is getting onto a horse as fast as possible, usually with the horse going at a full gallop. American Indian boys would actually practice this, and I'm guessing Mongol warriors as well. Untrained, but panic-stricken rider would probably do it out of desperation.
He receives a nasty slash on the thigh, but is able to steer the horse in the right direction, and lets it carry him full-pelt away from the scene. I doubt he'll have any directional control for a while, but it really doesn't matter. And it makes the escape a lot more haphazard.
The enemies don't have horses, so once he makes a nice distance he can calm the beast down, see to his wound, and continue at a more responsible pace.
That's sorta what I have in mind.