Along wth Angie's great reply
...when I brought a second dog into the house, the one thing I always make sure of is that my first dog is first in everything. Her food bowl goes down first, she is first in and out the door or car, she is first and last to get petted and first to get any treats. Other than having a slight problem with sharing her toys, the two are getting along great. I solved the toy problem by basically getting two of the same toy. This way, the older dog can't figure out wich one is really hers.
My pair are a 3 year old lab/chow/border mix and the new dog is (now) an 11 month old lab/chesapeake bay retriever mix.
Terese