Here is what I would recommend -
1) Create a new Simulation only model and put two pulse generators both with a sampling period of 0.02.
2) Adjust the square waves as you see fit and add them up and send it to a scope.
3) Simulate this model and make changes as you see fit.
4) Once you have established you have the square wave of your choice, then bring it back into Arduino model and send it to the digital pin.