Again it depends on what year KDX your are talking about. The front disk on 93-94KDX200 and 89-01 KXs should be the same as should the rear disk. I'd expect the 89 to 92 disks would be the same as well? There is a difference between the 95+ KDX disks and the KXs. I'm not sure if the difference in the bolt pattern or the thickness of the rotor though? From what Sage is saying it sounds like the difference is in the bolt pattern.
White Brother, and I'm sure others, have on line catalogs. Your best bet is to use them as a cross reference when you have questions about parts interchangability. You could look at www.buykawasaki.com as well but Kawasaki's part numbers are going to reflect small changes that may or may not effect interchangability. Things like difference cut outs in the rotor etc.
David