Let the college notes war begin

That wouldn't make them take more space, there would be no reason to say it.

When you print a document, you can print it "two pages per side". It basically divides each side of the paper in half and prints a page on each half. I think that's what he meant

If you print something 2 pages per sheet and double sided. You fit 4 pages of a document on 1 sheet of paper (page 1 front top, page 2 front bottom, page 3 back top, page 4 back bottom). So by saying "i have these 20 binders stuffed completely full with paper, all of which is double sided and 2 pages per sheet" its saying "this is 20 binders full, but it could have been 80 binders full if I had just printed out 1 page per sheet like most people default to"

I don't think he ment he put 4 pages on one page. A paper can only hold so much readible information before it's too hard to see. I can put 10 pages on one page, but that would be useless, just like putting 4 pages on one page, you would print out the extra pages so you don't stress your eyes. I think he's saying that's how many ASSIGNMENTS (exercises) he had, at 2 sheets of notes for each assignment, each sheet printed on front and back, that's a lot of assignments he had to do to make that many binders.