You first have to set a screen mode with the XSCREEN command. It takes a parameter, which is the number of the screen mode. Consult the help entry for more detailed info. You'll probably want to use XSCREEN 3. To change the current display to the touch screen simply do DISPLAY 1. Change back to the top screen with DISLAY 0.
The simplest way to overlap sprites is using the Z coordinate with SPOFS.
You need to specify the amount of sprites and BG layers that each screen will have in the XSCREEN command.
Both screens share 4 BG layers, as well as 512 sprites. Make sure to distribute them accordingly. The XSCREEN command takes this format...
XSCREEN <Mode>,<Top Screen Sprites Assignment>,<Top Screen BG Layer Assignment>
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