To switch which screen you are using, use DISPLAY.
To use the touch screen, use DISPLAY 1. For the top screen, use DISPLAY 0. Other than switching screens, it's the same as using the top screen.
LOCATE 5,5:?"HELLO!" DISPLAY 1 LOCATE 9,9:?"I'M DOWN HERE!" DISPLAY 0 LOCATE 0,0:?"BACK HERE!"