Thanks for everyone's replies! I found a yorkie on craigslist, and he came home with me today for the first time. We're trying it out for a few days to make sure everything works, but he seems to fit right in. He and Luna have already been playing together quite a bit, and she seems happy to have the company.

Luna didn't really "get" the kong at first, but once she realized that she could get the treats out, not just smell them and carry the kong around (LOL!!) she really took to it. I think that's helping too...