Bringing a new puppy in the house is an exciting time for everyone in the family. Unfortunately, it could prove to also be an intimidating, anxious or exasperating time for your older puppy. Even though your older puppy is still a puppy to you, in your new family dynamic, he will be the dominant dog. Because dogs, when living with other dogs, organize in a pack structure, it is important that the people operate within this structure by respecting the older dog's authority.
Introduce the dogs to each other slowly. If the first introduction can take place on neutral ground–like a park or a neighbor's yard–as this keeps the older dog from feeling immediately defensive.
Separate the dogs. Since it is not always possible to introduce the dogs on neutral ground, or gradually over a period of time, separating them may be your best first step. You can do this by putting the new puppy in a room that the older puppy doesn't often use, like a bathroom. Put a safety gate on the door and allow the dogs to sniff each other through the gate. A crate will also work for this purpose. If there is any growling between the dogs at all, give them a firm no and whatever your preferred correction method is. You must make the dogs understand that growling is not acceptable.
Feed, train and play with your puppies separately, particularly at first. It is important to keep any jealous behavior to a minimum. When the puppies do interact, be sure that they are both under control. You can have a family member or friend (an adult or teenager, not a younger child) hold the smallest puppy and bring the older puppy to him to sniff. Again, correct immediately if there is any growling or snarling.
Relax, but show your authority. Your puppies will pick up on it if you are anxious at all about the meeting. Be sure to let them know in every meeting who is in charge. If there is the slightest bit of jealous or aggressive behavior, you need to make sure you are stepping in and correcting. Being consistent is key here.
Keep your puppies play time to 5- and 10-minute increments at first. As they get to know each other, gradually allow them to play and hang out together longer. Make sure you play with them equally. A new puppy is cute, but don't forget the old one. You'll create more jealousy and potentially bad habits in the older puppy if he feels he is being ignored.