I'm installing Win XP on a new SATA drive following RAD's partitioning strategy. However he doesn't mention how to boot from a logical partition    
with drive letter other than C. I assume this requires a boot manager to handle NTFS. Any help please.

Putting all your eggs in one basket is a bit foolish to say the least.

Both OS's will run better on their own HD's.  And if one drive crashes or gets messed up, you don't loose everything.
Each drive can also be used as a backup for the other drive.
It helps if both drives are set up with TWO partitions....one for the OS and one for Storage.

Then you can pick several different methods of booting into one drive or the other.

I've done this several times over the years, using Boot Magic and other methods to select which OS to boot into.

On my PC, F11 brings up the boot menu at first boot up.  From there I can select any bootable drive on my system to boot the PC.
I can even boot to an installed Flash Drive or SD Flash Card, CD, etc.

I use the Vista drive to hold my Ghost backups of my XP drive and Vise-Versa.

Using that scheme, either drive can totally crash and I don't loose a thing,
with the backup on the other drive.  
It's just a little hedge against disaster.  I like that!

Cheers Mate!
The Shadow