While both CDE and the IRIX desktop were built using the Motif toolkit, they are two very different beasts with very different desktop metaphors, configurations, code, and essentially no overlap at the desktop application level. Both were first released within a few months of each other in 1993. (Note: it was possible to install CDE instead of the IRIX desktop on SGI machines, but that was a relatively rare scenario. Perhaps that is what you are remembering?)