Malbim: Kalev received a tremendous inheritance - Chevron and its many villages and fields. It was proper that he give [Arei Leviyim ¹ ] like a quarter Shevet. He gave Chevron, but kept the villages and fields. Now it says that the rest of Yehudah and Shimon gave eight other cities, like is proper for two Shevatim (each Shevet gave four). Even though most of the eight were from Yehudah, Yehudah and Shimon are considered like one [double] Shevet.