I would make an effort to work with the older dog even more. Have him on the leash as you move about the house and revive your relationship. It is the nicest gift you could give him at this stage of his life.
Teach him some fun new tricks, engage his mind, which will feed his heart and soul.
When you are directing a firm 'no' or 'stay' at the new dog be sure you use the new dogs name, then say 'good' and the older dogs name in a happy tone. Be sure they each feel the words you are saying by changing your tone, energy and even your facial expression. They will learn to be clear as to whom you are speaking.
We like to practice talking to a group of dogs by acknowledging them one by one and having each of them do something different. This way they are well rehearsed at listening for their name and the direction.
Love Them & Lead Them,
~Elizabeth & Doug
Dog Training the Way Nature Intended