Practice with a helper in the field at first, the ponied horses nose should stay around the shoulder of the leading horse, if they get eye level they’re more likely to race, and if they drag too far behind are more likely to get kicked or whip across to the wrong side of you and a lead rope across your back or under the lead horses tail can go very wrong very quickly.
Never tie the ponied horse on, if things go wrong and you feel at the point of no return, let them go. The loose horse will generally stop and wait if the lead horse remains under control and you absolutely do not want to get pulled off as you’d land between the 2 horses.
It’s best if your lead horse is able to be ridden one handed, you can ride 2 handed with the lead rope in the left hand with the rein, but if the ponied horse drags back or pulls forward you need to be able to effect their lead rope independently of the lead horses mouth so have to switch to one hand. I prefer to have a 10-12 ft lead rope, to give me some wiggle room if they mess about, I find a standard 6ft rope too short and I’m forced to let go more often.
Chances are a polo pony has definitely ponied in the past, so shouldn’t give you any bother. But it might take a little while for sid to learn either role, most horses aren’t used to working with another horse right in their bubble, that’s why the field practice is essential.