The magnetic material on both sides had a very practical reason to be workable in all kind of floppy drives.
For example the write/read-head in a C64 floppy was placed on the bottom, so the down side of the floppy got magnetized. But the head of the floppy drive from a Atari 800 was placed at the top, so the upper side got magnetized.
So basically, when a floppy was sold as 1S (single sided) this was pure marketing. :) At least for the most floppy drives.
There's also a second thing, the "index hole". On real 2S floppy there are 2, one for each side. But most of the drives didn't care about this index hole. So, this wasn't relevant for C64 users and such.
For example the write/read-head in a C64 floppy was placed on the bottom, so the down side of the floppy got magnetized. But the head of the floppy drive from a Atari 800 was placed at the top, so the upper side got magnetized.
So basically, when a floppy was sold as 1S (single sided) this was pure marketing. :) At least for the most floppy drives.
There's also a second thing, the "index hole". On real 2S floppy there are 2, one for each side. But most of the drives didn't care about this index hole. So, this wasn't relevant for C64 users and such.