The rules card from
Munchkin: Reloaded, as I mentioned before, covers that, but it restates a rule from the main rules, essentially: You may only have one copy of a Class Enhancer or Race Enhancer on any of your Races or Classes. I.e., you can't have a two Dark Races or two Elite Classes. It is a known oversight that the rules for Class/Race Enhancers are missing in the
Munchkin 8 rules.
All of the rules can be found on this page:
http://www.worldofmunchkin.com/rules