Either straight co-op or straight solo. You can't mix and match, unfortunately.
I am guessing co-op is alot easier though right?
I found solo easier. Even though you restart the level if you die. It's easier to keep up with just your own actions instead of someone else's too. You could also do a really fast "save & quit" if you die and keep trying it that way. I imagine it would take forever with that method.
GT: Incredigasmic Finished Pokemon Black National Dex @ 85 Hours and 15 minutes on 04/08/11!