The fact that this is affecting both of your chickens makes me think this is hereditary or related to nutrition or their environment. I believe some additional information would help people on here figure out what is causing this. Can you please answer the questions listed in the sticky at the top of this section? I copied them below for you.
Welcome to BYC, BTW
1) What type of bird , age and weight.
2) What is the behavior, exactly.
3) How long has the bird been exhibiting symptoms?
4) Are other birds exhibiting the same symptoms?
5) Is there any bleeding, injury, broken bones or other sign of trauma.
6) What happened, if anything that you know of, that may have caused the situation.
7) What has the bird been eating and drinking, if at all.
8) How does the poop look? Normal? Bloody? Runny? etc.
9) What has been the treatment you have administered so far?
10 ) What is your intent as far as treatment? For example, do you want to treat completely yourself, or do you need help in stabilizing the bird til you can get to a vet?
11) If you have a picture of the wound or condition, please post it. It may help.
12) Describe the housing/bedding in use.