The following is off the top of my head and is a "wish".

Maybe when you are doing "darcs record" and you are looking at a hunk, and you
see that the hunk contains two logically separate changes, you can hit the key
to split the hunk, and then it will show you a new hunk (replacing the one that
you were just looking at) which is the first half of the one that you were just
looking at.

That might be sufficient!