From what I have been told (someone who double majored and has been through the process) is that it could give you something to talk about in the interview, especially if your second major is in something non-science or something that really interested you. It might also show some hard work/dedication to academics.
All in all, it's not going to make up for anything lacking in your app (gpa, mcat, etc.), so only do it if it's something your passionate about, not to get into med school.