Here's another way to do it:
Color your object in the first color, then open the Appearance palette. In the fly out menu, select Add New Fill, choose the second color, then check 'Overprint Fill' in the Attributes panel. Finally, make sure View > Overprint Preview is checked.
The beauty of this is that you can select either color in the appearance palette and adjust the percentages. You can also drag it onto the style palette and reuse it throughout your document.
Incidently, I've never had problems with overprint in Illustrator. Quite the opposite - I've seen plenty of files with white set to overprint, and designers wondering why nothing came out!