... All of this is, as Tempodi said above, to circumvent QB64's own limitation of a single-window approach.
True, I just wonder why it shouldn't easily possible to implement real multi window support. I mean we already have all we need, we can have multiple images using _NEWIMAGE, _LOADIMAGE etc. and we can designate where to read/write from/to using _SOURCE and _DEST. All we need is another "canvas" (window) to display our images in.
Expanding the SCREEN command with another optional parameter wouldn't even break compatiblity with legacy code, which simply doesn't use the optional argument:
eg. SCREEN {mode%|imagehandle&} [, , active_page, visual_page] [, windownum%]
Although I've no idea how complicated it would be to manage multiple OpenGL surfaces internally regarding _DISPLAY, _AUTODISPLAY etc..