A Maltese is one of the best "second dog" choices due to their incredibly loving and tolerant nature. My parents have "adopted", over time, several of our retired show dogs. Invariably you are asking one crochety older dog to share its home and masters with another, younger dog, but one that is every bit as crochety.
We have had the best results with taking the incumbent dog or dogs out of the household and having them come home to live with us for a week or so and allow the incoming pet to have the run of the household and "bond" with its new owners. Then, we've reintroduced the rest of the family back into the home and with the scent of the newer dog everywhere in the house and the newer dog feeling more confident that it "belongs" there we have had very good results.
There is a great deal of dilligent supervision that is required anytime you
bring in a new dog to a home where one already lives. Big differences in
size (different breeds) or ages requires that you NOT allow them to "play"
unsupervised until you KNOW they can be trusted together !!!!
Larry Stanberry - Divine Maltese