Let´s ignore the fact that Kenpachi never actually used both hands when he fought Ichigo. Heh, that was the only realistic "power up" to ever appear in Bleach and you diss on it because of it´s simplicity.That kenpo thing was just invented at the drop of hat just so he could win the fight. He fought Ichigo full force. leave the possibilities to statisticians!
Well, guess what? Using both hands does makes you swing harder.
If you want deux ex machine moves, go take a look at the Sharigan.