I have a home daycare, and have three boys of my own, 4, 2.5, and 8months. It is amazing how much children learn from eachother.
I would definitely tell the teacher your concerns so they know there is a problem and you care. They might not even know there are parents paying attention to the webcam and it might improve their care! If they know there is a problem, they can do something about it. They might keep a better eye on the twins or work with them to change.
Be consistent with your child. Tell him hitting hurts, and don't let him have his way if that is what he is trying to do to get things. When he sees it doesn't work for him, he should stop doing it.
Part of it is probably a communication thing.. 18 month olds definitely don't have the greatest communications skills yet, and he's trying to get his point across, but he probably picking up some of it from frustrations at daycare too. Work with him to improve his communication, remain calm, respond to his needs without spoiling, and good luck!