Adopting a second dog soon, going to have some issues with the first?

My first dog is a pitt mix that we adopted two years ago, he is approximately five now. He is a needy dog and when other dogs are in our home he seems to get depressed and acts out by pottying inside even though he loves playing with other dogs outside of the home and has many doggie friends that he plays with in their homes and at the park. I know there are going to be some issues when we bring the new dog into our home (a cocker spaniel that is approximately five years old, so they are about the same age) even though when visiting the shelter they get along great. What is the best way to make my first dog comfortable with sharing his home and his humans? How should I respond to the potty issue? My current plan of action is to just treat him like a puppy by taking him outside every half hour so he doesn't have the oppurtunity to potty inside and keep all the doors closed other than the room we are in with him; is this a good plan?