Actually Carnage comes from Spiderman and and Venom after spiderman puts Brock in prison and is rejoined with the Symbiot... Thats why when Carnage fights either of them he calls them his parents. They can't put venom in the series till they introduce Brock and make him hate Spiderman... That alone would take an entire movie.

Having both carnage and venom in the same movie would be stupid and near impossible due to the length the movie would have to be because A) He gets defeated by both Spiderman and Venom and B) Venom must already exist and have foughten spiderman already and turn good.