Why not actually do it the way a two projector setup would.
Load slide 1 in projector 1
turn on projector 1 lamp
load slide 2 in projector 2
wait for slide advance
turn on projector 2 lamp
turn off projector 1 lamp
load slide 3 in projector 1
wait for slide advance
etc...
#include <WindowsConstants.au3>
$form=GUICreate("Test",900,530,518,272, BitOR($WS_SYSMENU,$WS_POPUP), 0)
$PSimage1=GUICtrlCreatePic(@ScriptDir & "\image1.bmp", 0,0,900,530) ;load slide 1 for 'projector' 1
GUICtrlSetState($PSimage1, @SW_SHOW) ;show 'projector' 1
GUISetState(@SW_SHOW, $form) ; start showing it
$PSimage2=GUICtrlCreatePic(@ScriptDir & "\image2.bmp", 0,0,900,530) ;load slide 2 for 'projector' 2
GUICtrlSetState($PSimage1, @SW_HIDE) ;hide 'projector' 2
MsgBox(0, "slideshow", "slide 1") ;wait for slide advance
GUICtrlSetState($PSimage2, @SW_SHOW) ;show 'projector' 2
GUICtrlSetState($PSimage1, @SW_HIDE) ;hide 'projector' 1
GUICtrlSetImage($PSimage1,@ScriptDir & "\image3.bmp") ;load slide 3 for 'projector; 1
MsgBox(0, "slideshow", "slide 2") ;wait for slide advance
GUICtrlSetState($PSimage1, @SW_SHOW) ;show 'projector' 1
GUICtrlSetState($PSimage2, @SW_HIDE) ;hide 'projector' 2
GUICtrlSetImage($PSimage2,@ScriptDir & "\image4.bmp") ;load slide 4 for 'projector; 2
MsgBox(0, "slideshow", "slide 3") ;wait for slide advance
GUICtrlSetState($PSimage2, @SW_SHOW) ;show 'projector' 2
GUICtrlSetState($PSimage1, @SW_HIDE) ;hide 'projector' 1
;GUICtrlSetImage($PSimage1,@ScriptDir & "\image5.bmp") ;load slide 5 for 'projector; 1
MsgBox(0, "slideshow", "slide 4") ;wait for slide advance
Exit
No flash on my system when I tested it.