Stroke after carotid endarterectomy is a fairly common complication of the surgery. However, it should almost always affect only one side of the body. There are a few exceptions to
this. Sometime, the carotid artery on one side of the neck will supply blood to both sides of the brain, either because of an inborn variant of artery anatomy, or because of an unhealthy artery on the other side of the neck. Another exception would be if the stroke was in an entirely different artery from the site of the surgery.
It sounds to me like the first order of business here should be to find out whether or not the post-operative strokes were on one or both sides of the brain, and whether they were in the artery where the surgery was performed. Your surgeon, and probably your internist, should know the answer.
If the strokes are only on one side of the brain, in the territory of the surgery, then it does not make sense that your father would have weakness on both sides of the body, since each side of the brain controls the opposite side of the body. In this scenario, other causes of weakness would have to be looked into. These would include such issues as 1) side effect to a medicine - particularly cholesterol or blood pressure medicine, 2) infection such as pneumonia or urinary tract infection, or 3) post-stroke sedation.