I concur with “really difficult.” I’m in the middle of testing Part 2 of a series and it’s a lot more complicated than I thought. Not only do you have to make sure everything mechanically and plotwise works within the new installment, you have to consider everything that happens in the context of the larger story. You also have to figure out how variables carry over from Part 1, and what’s good/bad/okay in terms of variable scores, so you can keep going with reasonable stat checks. You also need to figure out what variables to just ignore moving forward, because you can’t keep up with “remembering” everything that’s happened through the code.
CoG frequently uses the arm/hand analogy when describing what an interactive novel might look like. The arm is like 75% of the story where things stay fairly linear, and then it branches all over the place in the last 25% or so, like the fingers of a hand.
With a two-part series, I think you have to consider Part 1 all arm. And with my trilogy, I have to consider Part 2 as all arm as well. If you start sprouting fingers in the middle of a series, I’m not sure how you ever manage to finish. For me, the fingers will all be in Part 3.