To handle multiple dogs, separate them when they're learning new behaviors, at least when they’re learning the fundamentals like sit, down, or stay. Work with only one dog at a time and place the other dog(s) in a different room or in their crates. While you focus on the working dog, give the other dogs something to do, so they don’t fuss or feel resentful. Keep your training session short by making it something you can accomplish in less than 10 minutes. Once you’re finished with the first dog, swap them out for another dog. Repeat until all of your dogs have received their one-on-one training time.
If separating the dogs isn't possible, use a leash or tether to keep them in the same room without interfering with each other. Either will prevent the dogs from interfering with each other or stealing each other’s treats. Tethers and leashes also allow you to control which dog is getting rewarded for a given behavior while training two dogs at once.
Another approach is station training, where each dog has their own station or area to relax in while you work with one dog at a time. The dogs wait voluntarily at their station until you release them, which helps them learn patience and emotional self-control. Reward the dogs at their stations heavily and frequently for calmly waiting their turn. Rotate among the dogs so each one gets a chance to be “the center of attention.”
Teach name recognition to help each individual dog respond to your cues, even when their housemates are around. Use your body language and make eye contact, and teach your dog to recognize their name, then use it before any cue. If you have a large group of dogs, it can also be beneficial to teach them a group name that means “everybody listen.”