If you're involved in the retrocomputing or PC history scenes, chances are you've heard of double-sided floppy disks that are formatted for one system on side A and another system on side B. For example, I've got a copy of the game Ninja which had the C-64 version of the game on one side and the Atari port on the other. At the time this was a pretty straightforward thing to do because drives only read one side of a disk at a time. A couple of weeks back, PC historian Trixter came across a highly unusual 5 1/4 …