An 800k drive can be made to work as a 400k drive (from double sided to single sided). I didn't inspect the cable but if it's similar to the Lisa, I seem to remember something about removing pins 9 and 20 from the drive cable to make an 800k floppy work like a 400k floppy. You can't do the opposite. The Mac 128k doesn't support 800k disks in firmware so if your emulator emulates an 800k drive, that would be the problem, I think.

Uniserver will chime in, I'm sure.