You could also make 720k DSDD disks into 1.44M DSHD disks by making a hole in the corner (similar to the write-protect hole but opposite corner). Of course, 720k disks seemed to be made to lower tolerances than 1.44M disks, so it was not a very reliable method, and usable only in a pinch.
Good times in Jr High. Disks were expensive, but we found we could punch the 5.25" floppies in the school library to store copies of games on the back side. There were quite a few unremarkable software titles that got heavy usage.
I only bought expensive 3.5 disks in the 80s and all single sided (720) ones: i have 5 big boxes with them and most have a hole in them; I tried them all a few months ago and almost all still fully work.
Dad and I made a jig to do this so the hole was always in the right place. I remember doing at least 40 disks in one go, and I think most of them worked fine.
```just a word of warning, copy everything off them quick, as the 'side2' goes backwards to the cotton cloth cleaner inside the 5 1/4" case, so basically all the crap caught in it then end up coming out when playing other flip side :('''
I am also reminded of a game, I think it was Rescue On Fractalus that was manufactured with an Atari version on one side, and C64 on the other. I thought that was pretty clever.
