You mean that you want to drive Skopje-Mostar-Split in a single day? If so, that's a long drive that will easily take you 11-12 hours, if not more due to bad traffic, queues at border posts (total driving estimates at Google Maps are optimistic in the region). That means you barely have time to even properly visit Mostar on your way, let alone other towns.

If you do have a couple of days however to drive between Skopje and Split there are lots of interesting places (both cities, villages, and natural parks) though on your way to break up your journey. But it would be handy to know how much time you have exactly for the trip.