The concept happened way before that too. Even though the Atari ST and Commodore Amiga had different floppy disk formats (and were bitter rivals), because they shared their main processor (MC68000) a great deal of games and software were available for both.
Occasionally, that happened on the same floppy disk, via a horrific sector format interleaving trick invented by Rob Northen (who did a lot of copy protection stuff at the time). Notably, the Future Publishing magazines ST Format and Amiga Format started out as the combined ST/Amiga Format and the "coverdisk" was exactly that - readable by both, with different files on each machine.
This reminds me that one of the filesystems that could probably be added into the polyglot is Amiga's Fast File System, as the superblock is in the middle. Rumor has it that this minimised seek times from anywhere else on the floppy, but I would think that one doesn't need to read the superblock that often.
Occasionally, that happened on the same floppy disk, via a horrific sector format interleaving trick invented by Rob Northen (who did a lot of copy protection stuff at the time). Notably, the Future Publishing magazines ST Format and Amiga Format started out as the combined ST/Amiga Format and the "coverdisk" was exactly that - readable by both, with different files on each machine.