Fighting twin cats

Question
I have two rescue cats which I adopted just over a year ago. From what was told to me at the time, they should be 2 years old in January. They are brother and sister and have both been spayed. They were abused when first born -the female cat was trapped in a grill like cage and burned, the male had his paws broken. When they were initially rescued, they were separated for a time as the male had to have surgery (which only was successful for three paws, he has had the back right paw amputated)
I adopted them last August and they have always had periods of disagreement but it was always short-lived. They are generally affectionate or at the very least tolerant of each other (and of their humans!) They have a safe and secure environment indoors and during daylight hours live outdoors. Their outside environment is also pleasant for them with a garden and small safe neighborhood to play in.
Recently they have begun attacking each other. No blood has been drawn (as far as we know) but the squabbles are getting more frequent and violent. The male is slightly smaller than the female but has a temper. As you would imagine they had some issues with humans but these have passed with the male now trusting us more than ever. However when he is lifted recently (mostly to separate them) he screeches as if in pain, but we are being gentle with him.
Im getting very concerned as the fights are escalating and they are so loved.
I was wondering if there was anyway of reducing this behavior or if there was anything that could be done by us to help them?

Answer
Shelley Ann,

Often when there are sudden behavioral changes in a cat, it is usually due to a medical condition of some type or another.  I believe you should take them both to the vet to find out what is going on. The cat screeching in pain upon being picked up is a sure sign there is a problem.  When cats are in some discomfort, they appear to be much more short tempered as they hurt.

You might also want their thyroid levels checked.

Please let me know what the vet finds.

Concerned regards... Norm.