Where does the idea that these two are 'twinners' come from? Guaranteed, they had similar nicknames and served the same master towards the ends of their lives but apart from that, Quick was a vast and powerful man who was only brought low by physical injury and inability to cope with it. Donald, on the other hand, was a weakling who never had any power beyond that which Flagg conferred on him (except when he was starting a fire). Also, their personalities were different; Andrew was cruel and sadistic where Donald was only childlike and disconnected from the misery his fires caused other people. Donald could never have led a Luddite war faction, while had Andrew been stuck on the road with the Kid, he would no doubt have killed the Kid and stolen his deuce coupe in short order.
The phrase "My life for you" originates with Donald, not Donald and Andrew independently. Andrew only said it because Flagg asked him to. Flagg even admitted this, saying that the phrase had come from a former follower of his (Donald).