lots of suggestions, hopefully one will work to composite PDF? (unlike InDesign CS2 or Illustrator CS4 onwards, DCS would only work for Separated output from Quark)
1. Use AsIs for your output color space.
2. In Photoshop a 2 spot colour multi-channel .psd could be just 2 channels or CMYK+2, try both (cmyk channels can be empty).
3. In Quark use the psd import XT panel to place the file. (I have a vague recollection of the XT being re-written with 8.1, so upgrading to that or 8.5 might help also?).
4. In the old -old days the only sure fire way of achieving this was to 1st save the spot colour psd as Photoshop PDF, then in Acrobat Pro SaveAs EPS, then place the EPS into quark, (the preview may be worse than hopeless at this stage), then Print to composite postscript and Distill.
5. change versions of Photoshop, CS and earlier used whole number CMYK values to describe PANTONES, CS2 - CS5 used Lab and CMYK (not whole numbers) to describe PANTONES, CS6 use Lab values only. I've also noticed even custom named spot colours seem to retain a link back to a PANTONE name if that's how they were first chosen in Photoshop grrrr. Mix and Matching PANTONE Libraries in CS can cause Illustrator to be unable to open old files with placed spot colour files, if it can cause this chaos to an Adobe app I'm sure it is more than capable of messing with quark!