I'll try and explain it as best as I can. Either screen CAN show 3D but only 1 processer (ARM9) on DS can display the 3D image. The ARM7 (GBA processor) is used on the screen that isnt being used for 3D. Both screens can show 3D but the processor can only do 3D on one screen at a time. There are tricks with the GBA processor to make it semi-3D but it wont look as good as the ARM9 on the DS.
But me and Null are correct and have explained as best as we can
