Cooper was 10 months old when we got Carley who was 6 months old. Cooper was fully potty trained, I think that helped tremendously. They both got along really well together immediately. It felt like a lot more work for about a week, and then not at all after that first week. I would do it again in minute, Carley has been great for Cooper. He even went from a super picky eater to a good eater. I will always have two from now on.
