Default Restoring cwm backups of a different rom

This was something i tried to do, resulting in a brick, which i only got out of by flashing stock with odin and reflashing my rom.
basically, i flashed cyanogenmod, and made a cwm backup, but kept my stock rom cwm backup. i thought that by restoring either backup, i could "dual boot" my phone.
it didnt work. the phone went crazy, and soft bricked. im guessing that it has to do with the partitions, since cm10 and stock have different partitions.
weird thing is, i accidentally did the same thing on a friends phone, and it just (right after clearing all user data in recovery,) booted into stock rom.