One thing we always do when we dual-boot is modifying fstab and nothing changed on 64-bit devices. You still need to modify it to specify root partition and data partition (/private/var).
But before modifying anything we need to mount our second OS partition.
mount_hfs /dev/disk0s1s3 /mnt1
We need to modify to make it look like that:
Now we need to copy SEP to our second system partition