Zephonim, they've already proven they don't mind having villians cross over movies, with setting up for Harry to be a Goblin in the third movie at the end of the second. And I felt splitting it the way I did would allow them to tell the story reasonably close to how it should be told with Spidey wearing the symbiote for a time then discovering its evil, but first it has to seem like a beneficial sort of relationship. For the most part I only really have villians crossing over two movies. And as for Dunst leaving, don't care much, I don't think she's a very good actress, and I don't really like her as Mary Jane either, I also don't think she's all that pretty. She's pretty yes, but not that pretty. So if Dunst leaves I won't be that upset by it.
For the rest you just need to get actors to sign two picture deals, and maybe you don't even show them before hand. I just like the idea of setting up little side stories in the previous film, as they've sort of done, so we know where the films might be headed. If they want, they can not even introduce the Lizard until the forth movie, and not introduce Eddie until then either. And then not introduce Carnage until the last movie. I just felt crossing between two movies would allow them more of an ability to develop characters and things like that. But they don't need to, not really.