My personal thought is that orcs and dark elves probably don't run into each other terribly often - Not many dark elves live one the surface or in shallow orc caves, and orcs probably don't tunnel down deep enough to meet dark elves. I'm sure each god believes their race is superior, but its a lot easier to be nice to each other if your children aren't having wars over territory every five minutes. They both hate dwarves and humans and so on, so it seems to me that it would be a pretty workable alliance.
