WASD keys to control paddle on the left, and arrow keys to control the paddle on the right. To player 1, the paddle on the left, don't let the apple hit the edge behind you, and to player 2, the paddle on the right, don't let the apple hit the edge behind you.
This project and the original Pung 3 were finally fused! Check it out! https://scratch.mit.edu/projects/1046149870