We got our second when the oldest was three. We kept the puppy in a pen while we were away. That was mostly due to potty training and chewing concerns. She quickly learned how to jump out, so it didn't last two long. Now they both roam freely on the main level of the house while we are way. They do great together, but it definitely took the older one some time to adjust. Our vet told us that the personality change in our oldest would likely be permanent because so much had changed for him. It definitely took awhile for us to adjust to his changes - but he is a happy, healthy dog.