I haven't seen a double degree like mine very often, but I have seen the latter form on a LinkedIn profile where Googling for his name returned a university web site showing that the person actually had a minor, not a second major, in Mathematics.
It's true that degrees are not comparable and colleges have different standards of difficulty. Students may be able to convince advisers to count one course for multiple majors. And degrees are not a great measure of applicable skill.
However, if you want to get a degree, double degree, double major, or triple major, you should follow the procedures at your college before you graduate, rather than just adding it to resume many years later.
As another example, a classmate created his own major to take more Business and Psychology courses and fewer C.S. courses; he lists the degree like a triple major, which is very misleading.