Congratulations on your new little girl! I think having two is the greatest. Snickers was just a year when we brought Baxter home at 12 weeks. We didn't introduce on neutral territory because it was January and very cold, but it worked out fine. My breeder said to make sure we give Snickers all the attention he was used to, the new puppy wouldn't know the difference. Also, as someone above said, let the older dog be first with everything. Snickers let Baxter know immediately who the top dog was, and Baxter accepted it and they got along fine.
Today (4 years later) they are best buds and brothers. You never see one without the other. Also, Snickers had severe separation anxiety when he was the 'only' dog, but it completely disappeared when he had Baxter for company.
All in all, I think it's the best thing in the world to have more than one. Now I'm working on getting #3, just have to convince hubby.
