There are ups and downs. All I can do is share with you how I feel about my two pups.
Tina and Tbone are only two months apart, Tbone being older. We had him for 6 months before we realized that he was REALLY lonely when we left him alone. So, we got Tina, who was a little older when she came to us. Since then they have been inseperable, because they know that no matter what, they will always at least have each other. I can tell that they feel this way. They go through things better when they are together. So for that reason I am very glad I got a second dog. And I love little Tina. I think they also get along well because Tbone is a bit bigger than Tina, so he was always the boss. Also, Tbone is a momma's boy, and Tina likes my BF more, so they both get good attention and don't fight over mine. I don't know if that is a gender thing, but Tina is much more independent. I can't imagine having two momma's boys
The down side is this. They are harder to walk by myself, they are twice as difficult to take anywhere, in a car or a purse. Because of that I don't take them with me as often as I would if I only had one. Their vet bills and Rx medicine, food, toys, clothes, ETC... are twice as much. Also, the risk of health problems is twice as high now. Tina had some major health issues that we ended up spending way too much on to help her, and that would have been totally avoided had we never had her. But over that time, I realized how much she was worth to me, my BF, and Tbone.
I think folks on here will tell you that EVERY yorkie is worth it, you just need to know that you can afford the initial and long term costs and struggles with owning more than one dog.