If your older dog is generally sociable things will probably go well. â€­Be available as a referee during the first few days till you are comfortable. Showing simultaneous affection by sitting with both dogs, petting and cooing them together reinforces that there is no favorite pet. Feed them separately for the first week or two until the relationship is predictable. Try to keep as many elements of the older dogs routine constant so he feels comfortable that his situation is not in jeopardy.