The Rule of Thumb is 1.5% - 2% of the each horse's body weight. Being a new horse owner that's about impossible for you to judge.
Assuming (and that is a dangerous word):
The Paint weighs around 1,050 and Hafflinger around 850, you could take 2% of their combined weights and feed that -- yes that means you have weigh the hay; the easiest way is to stand on a scale with it.
If the horses are in together, make 4 or 5 piles of hay, at least 20 feet apart from each other so the horses don't fight. Horses don't always get along when it comes to food. If one of them is an alpha dominant horse, it will guard as much hay as it can to stop the other one from eating.
There's nothing you can do to change that mindset - it's been going on for a couple thousand years. That's the reason for more piles of hay than horses and for the piles being far enough apart that nobody gets kicked in the head.
That all being said, I just came up from the barn. My horses have been in since 5:30. The wind is howling like it did when I lived on the OH/PA border. The temps will drop to 28 so, with all that wind, the "Feels Like" is going to be just ugly.
Meaning, I loaded those horses up with enough hay to feed them thru Saturday
Like we need comfort food in this weather, they need more hay in this kind of cold/windy weather because they burn calories trying to stay warm.
Unless one of your horses really has a problem holding weight, it's hay you need to feed more of in the winter. Don't cut the hay back and up the grain, that's one way to colic a horse.
Do NOT rely on snow for water for them. I know of folks who think its ok to just let their horses eat snow and not give them water. Horse LIKE to eat snow but they HAVE to have fresh water, even if that means you're breaking ice 2 -3 times a day and dumping a few gallons of hot tap water in the tubs, if there's not electric for heated tanks.
They also need a white salt block beside the water. Horses can eat as much salt in the cold of winter as they do in the worst of the summer.
I hope this wasn't too confusing and that it helps