Having ran a rescue organization for a few years and having to bring some rescues into the home I have watched the interaction between our personal dogs with those that are passing through going to forever homes. There is always a period of adjustment and sometimes short tempers, but for the most part all goes well.
Old Fella, Mama, Peaches and Levi were always the ones to teach the others the ropes. Bringing in a new face for an older dog can be touchy if the older one doesn't want the territory to be invaded by another. However most likely your older dog will adjust to the new arrival and most likely will keep your older girl more active longer than if she were by herself. Mama was ten years old when she came to stay with us. Her owner died and left her without a home so we took her in knowing she would live out the rest of her days with us. Mama was a German Shepherd mix prone to have hip problems. She live another five years with us and I contribute her length of days to the fact Suzie, Hobo and Peaches kept her active.
There is a lot of good that can come from bringing in another dog to keep your older girl company. I say go for it and enjoy the days ahead.