We have a Shih Tzu that is almost three years old. I decided I wanted another as not only a playmate for Leo but another 'kid' for me.

I think if you have the time, money, energy and love, it's a great idea.
I also agree with the poster who suggested seeing how your current dog reacts to a strange dog in his house. Leo is really good with other dogs, so I'm hoping it will be okay. My only worry is that once he realizes the dog isn't leaving, he may get really ticked off.