Place the players right next to the green barrier on opposite sides, and measure their x values.
You could insert the following events, and this is assuming that Player1 is on the right side and Player2 is on the left:
Player1 >> X > (whatever the x value that you measured) | Player1 >> Set X to (whatever the x value that you measured)
Do the same thing with Player2, but make it "X < " instead of "X > ".
Does that help?