I'm in the same shoes this week. I have a 2 years old male pure yorkie and just brought home a 6 months female morkie. At first my yorkie (Yobo) was barking and growling at Boba. Sometimes jumping on her trying to nip at her. What I did is started to hold them both and pet them at the same time giving them both attention and gradually letting them get familiar with each other. Yobo shows signs of jealousy at first. I also trained them together like sit, shake, hi-5, and standard obedience. Provided both with lots of treats. This was great as Yobo already knows all these tricks so Boba was picking it up fast from watching him. At first Yobo didn't want to sit next to Boba. Gradually I moved them closer. Also, what help tremendously was that I take them for walks together at least 4 times a day. I went to the park and let them loose and had them both chase me. Walks together was great as it helped them bond and also Boba watching Yobo she learned how to be potty trained. In doors I throw a toy and have both of them fetch. They have minor scruffs but eventually they started to play together and even played tug with each other. It's been 4 days and they are great with each other now. They even play with each other when I'm busy. With Boba it helped Yobo with his separation anxiety as well.
The key is have them do activities together and give them LOTS of treats. Also, give the original dog a bit more attention so that he/she knows you still love him/her. G'luck!
