The Hidden Power of Photoshop CS: Chapter 2: Color Separations. Pt. 2. By Sybex - WebReference.com-:
"A second way to achieve manual duotoning is to create the effect using spot color channels. This creates an image with the spot color built in, and essentially it will be press ready as a separation. In the following techniques you will use both the manual layer method just described in the previous section and Photoshop’s duotone interface to create duotone results."

http://www.webreference.com/graphics/ps2/