Are you in 1st year?
After 1st year you get to go on MUGSI again and pick what you'd like to double-up in. Then later on they will say which you are accepted or rejected in. You get 4 choices, double majors will be one of the choices available, as will single honours, and just a major. You get 4 and you should use them all up but sometimes, like me, you know what you want with only one choice. (I made my 4th choice a double major in religion and politics; I just didn't know what else to take and I like God.) However, you need to actually have certain courses to pick those majors, obviously. For example, I picked religion in combination with poli sci but never actually took religion--
. You can do inter-faculty double majors, especially in the humanities and social sciences.
If you got any issues ask advisors but it will be made pretty clear when the time comes.