A paper split in two parts - how to cite it in CV?
Option (a) does seem to be a better choice at this point in time. Later, when one of the parts are published, it would be better to list only the published article instead of the preprint. If you are concerned about the visibility of the other part of the work, you may upload a preprent of just the other part to make sure there are no overlaps in your publications list.

Once both papers are published it would make sense to only include those articles without the preprint entirely.