He does a lot of stuff even if it isn't series work, and not always in places you'd expect. For example, he does the cover art for the Venture Bros DVDS
I don't belive that in Nite-Owl they ever say that Rorshach and Dan are partners...
I don't remember if they come right out and say it but the story is written so that pretty much every time Dan is in costume he's working with Rorshach.
It would be like if "Batman: Year One" had every issue be about his team-ups with Superman.