Paul,
This is perfectly possible within a single multi-part question. Create two inputs, one for each part.
Then you also need two potential response trees. "Tree 1" will only depend on the first answer. So, you can choose to give feedback/marks if a student only answers the first part. "Tree 2" will depend on the first and second answer, allowing you to use both parts and create follow through marking if you choose to.
STACK was designed with exactly this kind of thing in mind.
There is currently no way for the question to adapt, e.g. to only show part 2 when part 1 is correct. I've thought about this, but a general mechanism becomes very complex very quickly.
Chris