Complex sentence: Guitarist/keyboardist Mark Hallman was another musician considered for Firefall but Hallman turned down the offer to stay with his band Navarro, who went on to back up Carole King.
Split into simpler sentences: Guitarist/keyboardist Mark Hallman was another musician considered for Firefall. But Hallman turned down to offer to stay with his band Navarro, who went on to back up Carole King.