Film franchises can be tricky to pull off in multiple parts. Most sequels, even if they have a continuing story arc, come so much later than their predecessors that the payoff for a plot can be difficult to capture. There are, of course, exceptions -- The Empire Strikes Back led directly into Return of the Jedi, The Matrix Revolutions followed immediately after The Matrix Reloaded and the entire Lord of the Rings trilogy formed a coherent, overlapping story.
However, those are examples of major franchises that were more or less planned as unified stories. James Bond has always been about single installments, and any hope that this pattern would be changed was pretty much nixed by the critical and box office failure of Quantum of Solace, which continued the story that Casino Royale began.
However, there were rumors that the next two 007 pictures following the imminent Skyfall would tell a single story. Daniel Craig has basically put that idea to rest. According to /Film, he went on record as saying:
No, I don't know where that's come from. It's impossible to do a two-parter, I heard that someone was talking about that -- [that] we're doing a two-parter, but that's the first I've heard about it. We can only do them one at a time; they take six months to shoot. You can't write one movie thinking about the next. All we're trying to do is get the next one sorted out and it'll stand on its own, and if I'm able I'll do another one after that.
Anyone who was looking for an extended arc might be disappointed, but at the very least, this will make sure that every installment will be singular and coherent -- something that the Bond franchise has been lacking for a while.