Read the source, the Fullscreen backbuffer count is 1.
ddsd=New DDSURFACEDESC2 ddsd.dwSize=SizeOf(ddsd) fullscreen=state&DX7FULLSCREEN flipflags=DDFLIP_WAIT If fullscreen ddsd.dwFlags=DDSD_CAPS|DDSD_BACKBUFFERCOUNT ddsd.ddsCaps=DDSCAPS_PRIMARYSURFACE|DDSCAPS_FLIP|DDSCAPS_COMPLEX|DDSCAPS_3DDEVICE ddsd.dwBackBufferCount=1 Else ddsd.dwFlags=DDSD_CAPS ddsd.ddsCaps=DDSCAPS_PRIMARYSURFACE|DDSCAPS_3DDEVICE EndIf