Interestingly, the bottom screen takes the leftover sprites (512-# on top screen) and background layers (4-# on top screen). For example, if you did "XSCREEN 3,512,3", the top screen will get 512 sprites and 3 background layers, as usual. However, the bottom screen receives only 0 sprites and 1 background layer! If you use "XSCREEN 3,0,0", then the bottom screen will get all 512 sprites and 4 bg.