Using a second controller in place in the second controller port pause game using "Start" Then using the 2nd controller input L1,R3,R1,L3,Right,Square,Up,Square,L2,Left,Square,Down,R2. If done correctly the game will unpause and all the enemy will freeze. If you go up to an enemy now you can control them (limited) with the 2nd controller using it's "D-Pad" or you can stealth kill them without them fighting back. Be careful how you use the controls as some can freeze or crash the game.
(This is part of the debug code released by K2 to Tenchu.de)