Even though I don't really know for sure, you need to create 2 different partitions, one to put the PdaXrom filesystem and the other one to put the OZ filesystem.Then you need to tell the bootloader to prompt you for a choice between the two.
So, if PdaXrom is on ROM, and you want OZ on SD, find out how to boot from SD with your bootloader, and then add the commands for booting OZ in the bootloader configuration file (Which i think lies inside /boot).
Or not.