For some missions, Terry and Clay are automatically there backing you up. For these missions, both of them will gain 12% toughness. For the missions that you manually call them for backup, they will gain 8% toughness and for gang wars they will gain 4% toughness. If one of them dies during the mission, they will not earn any toughness for that particular mission. You can also call them for backup and once they arrive you can just drive around for a while and they will gain 1% toughness every minute.