Older dogs usually accept a new dog, but some struggle. Let the dogs work it out. Step in only if the situation could become violent.
Two dogs: Does gender matter when you adopt a second dog?
Does gender matter when you get a second dog? The experts say yes, gender matters, and recommend getting a second dog of the opposite sex.
Is it better to have two dogs? 5 factors to consider first
If you plan to leave your dog during the day while you are at work, it may be better to have two dogs so that they can keep each other company.