Simply start a Quick-Play Game against your 2nd controller.
Play standard quarters if you want to stack with some other achievements. Otherwise change your quarters to 1 minute each
Keep scoring with Player 1. If player 2 gets the ball, just throw up a half-court or full-court shot to turn the ball over. And keep scoring with player one.
You should be up by much more than 20 points by the end of the game. The Achievements will pop once it ends
You will also get the "Make A Statement" Achievement for outscoring an opponent by 20 points in a game
Here is a Video Guide: