Are you sure you are supposed to take the older pup out and let the new one roam the house? I haven't heard of that and to me, it sounds intrusive to the older pup. But what the heck do i know?
What I've been told and what worked for us is having the 2 dogs meet on neutral grounds, meaning not in the house. This way they can feel comfortable around each other without one feeling defensive or uncomfortable.
When Max came into the house I made sure that we gave Cooper almost all of our attention. The key is to not let your current dog feel that he/she is not the alpha. You definitely don't want jealousy.