First of all bear in mind that I haven't had anything to do with Photoshop for the past couple of years but from Photoshop 5 it should not be too difficult to convert an image to spot color without having to import it in Illustrator.
If you can decently separate the two colors by just selecting them and creating alpha channels you can then create the two spot colors and fill the alpha channels with them.
You also have to save the image in DCS format.
If the yellow color is "superimposed" on the black (I'm talking here about how the logo looks like) I would also set the trapping, usually to .216 points.
To set the trapping you need to convert the document to CMYK temporarily, after that you can convert your file to grayscale, since there is no color information to be imaged in the primary channels, and changing mode will reduce the file size.
Hope it helps.
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