Ehh don't compare Billy with Shitva.
In Re0 you could actually control both characters and solve puzzles while the characters were away from each other. The difference is control. Re0 had it, RE5 doesn't.
I RE5 you have that annoying woman the whole time with you. I agree it's fun to play the game with a friend. But she's really annoying when you want to play the game on your own. She's just there to kill zombies for you. And open a few doors that you can't reach...
And I don't expet the game to have a much deeper co-op since Capcom already announced the game wouldn't have any puzzles in it...