Once I figured out the timing on this achievement, I got it within 3 exhibition games.
When you team has the ball, hold LT like you would for a normal alley oop, and press A when you partner goes up into the air. As the pass hits his hands, tap A again (or tap it a few times which worked as well for me) and he will quickly pass it off again. As soon as that happens, hold X until the shot meter gets into the green area, and then release X. If the timing is right, the original player who made the first pass will run in and dunk the second pass. You likely wont get it right every time, but its not too bad once you get on a roll.
I did this with Steve Nash and Michael Jordan, so while players with higher dunk ratings will make it easier, it worked just fine with Nash as the dunker as well.