Why is General Huynh Van Cao's chief-of-staff, Lt. Colonel Bui Dinh Dam, selected, even though he is a North Vietnamese and General Huynh Van Cao is a general for the South?

Lt. Col. Bui Dinh Dam is selected because of his Catholic faith (primarily) and for the fact that the Americans strongly sympathize with the cause of the republican government of South Vietnam. Also, General Huynh Van Cao knows he can control Lt. Col. Bui Dinh Dam.