Its a matter of preparation. Contadors fault was he tried to be in top shape in both races. Hence he was fatigued and wasnt at his Giro form when the Tour kicked off.Alphabet said:Contador couldn't do it. Wiggins has no chance.
Anyone going for the double should try to win the Giro by a small margin, in order to save energy for the Tour. And not crush your closest opponents chances like Contador did in 2011.