I used to do this heaps when I was a kid OP, it was the only way we could get the school ponies from their weekday agistment to their weekend place.
If you have a dodgy knee and are a bit of a wuss, may I suggest that you have a horse person on foot go with you, if you are still going to try this? Make sure that person is ready to grab the pony that is being lead if you run into trouble. At least it will minimise what could be a potentially dangerous situation.
Other than that, ponies learn very quickly what they need to do when being lead from another horse/pony . The two will find comfort in sticking together anyhow. Just make sure that you don't have one who marches on, and another who is a slow poke.
I wouldn't try it out first time on a hack in case things go wrong. As others have said they can have a kicking match or one can match on/refuse to move leaving you being pulled between the two. If you can try it in the field (or an arena) to get an idea of whether they already know ride and lead or not, and whether they get on. Also best to do it when they are in regular work and not full of energy.
It is easy as long as you have fairly polite ponies. I can lead both of mine from other horses, but the sheltie tends to try and wedge himself under whichever horse I am riding, then he will try to get ahead, then when he gets bored he tries to grab the reins/martingale of the horse I'm on, and if he gets really bored he bites at my foot. He shows all of this rather irritating behaviour if I am leading him in hand too though.
What I wouldn't do is ride and lead if neither have done anything for a while because they can wind each other up and then you are in trouble.
I used to do it a lot , and it works if they get on normally . I did once make the mistake of leading my first pony out from the back of my big ID hunter , but she was so outraged at not being allowed to be out in front as she considered her rightful place in life , that she spent the whole time nipping him just behind my leg , until eventually he stopped dead in the middle of Chobham High Stret and tried to kick her in the head .
I slunk home leading the pair of them .
It would be worth just checking with insurance company as to whether you would be insured doing it . I can see how some would argue re potential for accidents to happen .
I used to do it regularly. It's fine if the horses are fine. I couldn't do it with my horse and pony but regularly used to ride and lead from my pony a little hunting pony. Both were easy. My horse just hasn't got the personality to do it ATM. If you do try make sure you do it with someone around first and dont go too far!!
The ponies havent been ridden for nearly 2 weeks due to school and weather. I've got to get them out of their field before they go mad. Was thinking of riding the 13.2hh (which I've done before alone) and leading the 11.2hh (which I've never done). I'm quite a wuss and have a dodgy knee so my seat isn't as good as it once was. They are both good ponies generally. Other option is to take the 11.2 for a walk!