I have 2...got the boy first...he was 14 months old at the time. Then, we puppy sat for a Shih Tzu and when she left, he was so forlorn, we got Mia, who was 17 months old at the time although she is actually 6 days older than Brody. They are such good buddies I cannot imagine not having them together. People think they are litter mates they are so close. We take both with us, and we even take both when just 1 of us goes. We are again puppy sitting the Shih Tzu, the gorgeous ex-show-puppy Sunny, and we took all 3 with us today...they get along so well together. My older son brought his Lady (a Doxie mix recovering..well!
...from back surgery) and all 4 had such a ball playing together.
I would do it again in a heartbeat...bring in a companion/playmate for a single Yorkie (any dog or cat, actually). The adjustments are minor in my experience compared to the love we receive in return.