You need to define a label on the internal and another one on the external screen in the setup. In your screen update function (or your main loop) set them to the same value (or variable) and update both screens. As long as you use pre defined labels it is easy. It doubles the code for updateing but fix the issue. If you want to draw lines or print free text you have to deal with the different resolution an aspect ratio and calculate the coordinates. That require some smart thinking but is still far away from witchcraft.