Riku being 'Dawn' aside, there are no true affiliations of Dawn and Dusk. Riku's using the Darkness for 'good', or to fight against other beings of Darkness. Dusk would be using the Light to fight other beings of Light, or something of that sort. However, that doesn't make for a whole new affinity. Both fall under Twilight, yet that as well isn't an affinity.
Well it's true that dawn and dusk are both twilight.
If one path leads to darkness, and the other path
leads to light, then the two other paths would act
as a sort of border between the two sides. These
paths, being between the darkness and light, would
be the twlight paths. Of these two paths, the one
that has the sun rise behind it (as seen in CoM) is
the Road to Dawn, and natually the side opposite
of this path would be the direction in which the
sun sets, thus being the dusk path.
I don't see why Dawn, Dusk, or Twilight for that
matter doesn't qualify as a sort of affinity. Are
you saying that Darkness and Light are the only
affinities in the series?