DS Swap Display Assembly code

[habababa]

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?

[zeromus]

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.