I've never heard of this being used for hybrid C64/PC discs before, but the technique itself isn't all that new - as far back as the late 1980s there were dual-format Atari ST and Amiga discs which would work on either system. Some were mounted as cover discs on magazines, and indeed at least one magazine managed a three-way disc that was ST, Amiga and PC compatible.
Some released software was dual-format as well - I particularly recall the Atari ST/Amiga version of Starglider. While the ST and Amiga boxes were both separately labelled, the actual disc inside was the same, and worked on either machine.
ST, Amiga and PC all used the same magnetic encoding, called MFM[0], so differences were completely logical (and not even very different - e.g., the ST and PC both used the old CP/M "FAT" format that is still with us today). That was more like having a file that's at the same time a .c program and a shell script. Nice, but at a rather high level of abstraction.
Apple II and the C64 used GCR[1] encodings (not the same GCR encoding), which is completely different. A C64 drive controller cannot read MFM, and a PC disk controller cannot read GCR.
The interesting thing about this hybrid disk is that whoever made it alternated the tracks - odd were GCR, even were MFM, and the logical filesystems were arranged such that each file system respected the other filesystem's tracks.
I bought an Atari St 1040 something in college, with a dot matrix printer. I told no one, except my girlfriend, about my computer. I literally felt like I was cheating. I only used it for word processing, but it saved so much time. I don't think I would have graduated, if I didn't have that Atarti St?
When I tried to sell my girlfriend on the computer; she just wasen't interested. "I will pay someone to type my papers. It's not that expensive. I got the brilliant notion, I would play to her creative side. I opened the paint(forget the name of the graphics program), and drew an erotic picture on the screen. Well that sealed the deal, "Put that computer away, or you will never get to see my erotic parts!" Never again told a single person about my cheating devise. Threw it away after a nervous breakdown in graduate school. I still miss that dot matrix printer--that lasted years on the same ribbon? I think I bought maybe one ribbon, and it was cheap?
Some released software was dual-format as well - I particularly recall the Atari ST/Amiga version of Starglider. While the ST and Amiga boxes were both separately labelled, the actual disc inside was the same, and worked on either machine.