I’m studying at a CUNY school and we have many double majors. It’s quite simple, actually. All of you have to do is allot many of your elective credits to fulfill the requirements for a second major (usually about 24 credits). This also eliminates the need for a minor which is usually 9-12 credits. So your breakdown would look something like this:
Out of 120 credits:
60 Credits, Core Curriculum
25 Credits, Major 1
25 Credits, Major 2
10 credits, Electives
(If you didn’t double major, you’d use about half of the second major credits for a minor, and the other half as electives.)