devkitPro

I'm using ASM to code my homebrew app. using GBATek as my reference.
and I'm making my own asm display swap code.
By setting bit15 of 0x04000304 to 1 in RAM,the dispay of engine A is sent to the upper display.
If I swap displays by clearing bit 15, do I have to force VBlanking?

If I need to,which register do i need to change?
Setting bit 7 in RAM address 0x04000000 forces a VBL and allows access to VRAM during the VBL.
Bit 0 in RAM address 0x04000304 also does the same function.Clearing bit 0 turns off the display.

Which comes first?Force VBL or Swap display?

have you tried just swapping the displays and seeing what happens? nothing else you said really made sense to me. I don't think any of it is related.