To do the double Musou attack in one player mode, your bodyguard has to have full Musou. To tell if he has his Musou bar full, look at your Musou bar. If it is flashing green, it indicates that you and your bodyguard have full Musou. Go near your bodyguard and press Circle. It should show your character and his bodyguard back to back and performing a true Musou attack. This attack is the equivalent of two people doing a true Musou x2 attack at the same time.
Back to Dynasty Warriors 3: Xtreme Legends