EasyBCD will indeed make it easier.
Specifically, ensure that the system always boots to your later install (esp. if you have XP as your 32-bit system) by adjusting the boot order in your BIOS. I strongly recommend that you plug the drive you want to boot off into the very first SATA slot; otherwise, if you plug in and remove devices the boot order seems to shift.
Now that you are booting from the drive with the 64-bit OS installed, use EasyBCD to add the 32-bit OS to the 64-bit OS's boot menu.
A question: Do you have two drives (sounds like it) or one with two partitions?