I think Princess Diana's advice is great. I had Jake for a year when I decided he needed a little brother.
My concern was that Jake would be jealous. He is very very laid back, is very quiet and calm most of the time. He's very playful, but he is just so good.
He's not territorial, and he's not agressive at all, but VERY attached to me. We took Jake with us when we went to get Jackson, so they met on neutral territory.
I'll be honest - Jake was NOT happy at first. In fact, he seemed mad at me. He wouldn't let me hold him, wouldn't even look at me! But, he was fine with my hubby. Thank goodness that only lasted a couple days, and then he was fine. The two had a great time playing, but Jake has certain toys that he still won't let Jackson play with (it's a year later). He doesn't fight with Jackson, but he will just growl, and Jackson will walk away.
Jackson is very different than Jake. He's really hyper, barks alot, and is more aggressive. It has taken ALOT of work to get him under control. He has improved a great deal, but it's an ongoing thing. I'm lucky to be with my boys all the time, so I can keep him in line. He is at the point now where he will listen to me, and he is a little less hyper now that he is older. He is still VERY jealous of Jake though. Jake never seems jealous, isn't bothered if I'm playing with Jackson or holding him. If I'm playing with Jake, Jackson will get in the middle. If I'm holding Jake, he will get up on my lap and try to squeeze Jake off. So, we are working on that.
My boys do like to play together, and they play all the time. But, Jake was the only pup in the litter, and he has always been very content to play with toys alone or with us. So, sometimes he just wants to play by himself, and Jackson won't let him. They have tons of toys, and some of their toys are identical, but they ALWAYS both want the same toy. Always! I can toss two toys when we play fetch, and they with both bring the SAME toy back together lol. I stopped buying two of everything!
Although they really do seem to love each other, they rarely lay together or cuddle. Honestly, I think Jackson really loves Jake, and Jake just "tolerates" Jackson. But, they don't like to be apart! If Jake has a vet or grooming appt. and Jackson doesn't, Jackson will bark the entire time and pace and fret. When Jackson goes, Jake will walk around whining. It's a love/hate thing! lol Also, they are VERY happy to see each other in the morning, which is funny because they both sleep with hubby and me. They get all excited, and it looks like they give each other a kiss and they even cuddle for about 3 seconds.
There are times when they both will cuddle together, but it's not very often at all. Usually, Jackson tries and Jake will move a little farther away or get up and move. Lately, I've noticed they seem to be getting a little closer so there's hope!
One thing I've found that has worked really well for my boys is making sure they get plenty of exercise. I walk them twice a day, and we get plenty of play time in. They love to play fetch, so that's a great way to get some exercise in. I spend time training them to do tricks, and working on some of the behavior problems that we sometimes have with Jackson, although we seem to have it down pretty good right now.
Getting plenty of exercise has changed Jackson's behavior in a very positive way. Also, my dogs understand that I am the alpha, not them. I'm strict, but I also spoil them rotten. I think you can do both, you just have to find the right balance.
I'm really happy that I have two. It's alot of work if the two have any issues with each other, but it's so worth it when you spend the time to train....or you can even take them to classes or talk to a trainer. I just preferred to try it myself first because I thought it would be a another great way to bond with them. They are soooo much fun, and I'd love to get another eventually.
If it's possible, you might want to try taking your first yorkie to visit the new puppy several times. That will help them get used to each other, and you will be able to see how they interact and get along.
Another thing that's good is that the two will keep each other company if you aren't home. Fortunately, I'm able to be home with mine all the time. I really don't leave them very often, but when I do, that's when they tend to seem closer to each other. (We videotaped them one time when we were gone for a couple hours -- they had a good time getting most of their toys out of the toybox, then they played for a little while and curled up together and slept lol). Of course, you may have to keep them separated until the puppy is a little older.
I'm sorry this is so long and wordy. I just wanted to give you my experience and how it has worked out. I've found the Dog Whisperer's methods to be very effective!
Congratulations on the new puppy if you decide to get him/her! How exciting!!