Maybe get one first if you must get them young. After the period of potty training is over and you've managed to bond well with your pup, consider getting an older pup or young adult already spayed/neutered. If you get an older neutered boy, it might be better to check that he was never used to stud before he was altered. If you're getting the boy first then an older spayed girl next, then you wouldn't need to worry about your boy sensing your girl in "heat" as she will have been spayed then he may never feel the desire to mark in your house. One can always hope.
When my Shih Tzu passed away in the fall of '07, I got my yorkie the following summer. We were still not quite ready for a Shih Tzu again. We did look, and we were open to having another Shih Tzu at the time, but it's difficult not comparing them all to our little girl and so we have yet to find "the one" for us. As for getting a yorkie, I had always planned to have one since well over 5 years ago. I just wanted it to be the "right time" when we would be ready to make a full time commitment to raising a little puppy. I had always wanted to introduce the puppy into our lives while our older girl was still around. But our timing did not work out.
Now I am also open to finding the next "right" dog for us so that my little yorkie baby can have a real friend to play with 24/7. Our cat makes herself very scarce as a part-time companion.