CMYK to Duotone/2 colour file?

Hi,

I've created a CMYK file for a business card and letterhead (in black and blue), but have been asked to convert it to a 2 colour file to save printing costs.

Is it possible to use the duotone function for this? If so, how do I do it? Also, on the letterhead, I've also designed a slightly faded logo as a kind of watermark- is it possible to alter the amount of ink that goes on to that particular part of the file?

Thanks very much for your help!
Natters