Seems like the author owned a ZX Spectrum and little else. There were other 1-bit machines out there, like the Apple ][. Paul Lutus' Electric Duet managed to

Seems like the author owned a ZX Spectrum and little else. There were other 1-bit machines out there, like the Apple ][. Paul Lutus' Electric Duet managed to get two-voices out of the speaker and he describes the process here: https://arachnoid.com/electric_duet/index.html https://arachnoid.com/electric_duet/index.html