Can somebody clarify something else for me? If I send a single colour job to print with a mono background image, everything will be printed as the choosen single colour, right? But with a 2 colour job, how do I specify which colour will be used for the monotone (photo) background image.... Do I have to assign the colour as multichannel in photoshop before placing into ID, or can I do this directly in ID?
--[If you output each color layer as a separation, you can tell your printer to print them whatever color you want. The problem comes when you understand that gray is going to turn into a percentage of the color you want to print (a screen of that color), rather than 100% of that color.
I would specify in your photoshop file what duotone you want it to be and set it that way in the file using "Duotone" color space. You used to be able to select a grayscale image in a program like Quark and add a color to it that way, but I think it's better to set a raster image's color in Photoshop and a vector image's color in Illustrator. Then you don't have to worry about what colors the InDesign contains, or having to choose pantone colors in InDesign.
This is how I do things anyway…
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