I'e had them do that. We had one eat his mother's tail off, and she had a long thick tail. It was so much I called the vet, I was afraid he would colic. or not be able to pass it.
Vet said he mmight, but doubted it. They do it from nervosness, so he said, but one can see another doing it and pick up the habit. Maybe that is why you have two doign it.
Bekcy
I can't think why they'd be nervous but maybe the weather was making them so. We've sprayed all the tails with a mixture of baby oil, cayenne pepper and black pepper because the bitter apple didn't work but I was just out a little while ago and my mare's tail is shorter than it was.