I have two Yorkies and two Biewers and when I added the second Yorkie, Cash had been an "only child" for more than two years. He was well-trained and socialized, but he didn't want anything to do with his new friend. It took a while, but now, three years later, they are great friends.
Respect your dog's boundaries and don't neglect to take them for walks together. Walking together is something that only friends and pack members do, and doing it with both of them will establish the idea in their minds. If they play, don't interfere too much unless one is getting hurt. Feed them together if you can.
It takes a while to bring a new member into a pack, but the fact that she is showing interest is a great sign. Keep giving her as much attention as before and let her get used to the idea. If she's friendly, she'll come around
