First, count nights rather than days. A night is always a night, a day can be anywhere from a couple of hours to a full day for sightseeing. Transportation in Sicilia takes time.
I would spend two nights in Agrigento, the archeological zone is extensive and there is a good museum across the street from it. Taormina would be another shorter stop. Siracusa and Palermo would be my main bases.
Ortigia is the old town of Siracusa, so no need to add both. Cefalu can be a daytrip from Palermo.
Catania is convenient to fly out of (no backtracking to Palermo), but if you mainly want to see Mount Etna, you can do that from Tairmina.
If you really want only two bases, you will have to give up something, perhaps stay in Ortigia and Palermo with some long daytrips.