My brother had my father sign power of attorney without my knowledge when he was in the hospital. I live with my dad and take complete care for him from his meals to his well being etc, My brother lives elsewhere & doesn't even call him to ask how's he doing or visit him at his house at all and my brother just lives 2 streets away. I'm the one who is there 24/7 and cares for my dad's every move, making sure he doesn't trip on anything or hurt himself. I found out recently how powerful a POA can be and I don't want to be left out of the decisions made for my dad's well being. My brother basically took charge without my knowledge for his own best interest and had my dad sign his life to him.
As long as he is still competent, your dad can change his POA himself with a notarized signature.
I don't see how having a medical Power of Attorney would be in the "best interest" of your brother. What is there to gain? Your brother would take advice from the doctors and make those decisions needed for your father. Sounds like your Dad is now back at home, no worst for the wear.
I am not familiar with any *form* for adding a second Power of Attorney name, just that the old Power of Attorney would need to be re-written to include all the names needed to make decisions for your Dad if he is unable to make the decisions himself.