It would be be "better" to install them on seperate drives, but what you would do when installing the first OS is format the "C:" drive doing a Custom Install, and then you would partition it before/during the install process at this time as well, assigning half the space of the drive to 32-bit windows and when you get to installing the 64bit version assigning it the other half.
And Drive letters only exist inside of windows, and windows will ALWAYS assign C: to the drive the OS is installed on, so when they are both on the same hard drive, BOTH will say they are installed on the C: drive, but they will be able to see the other partition (other windows) and will assign it a different drive letter.