Is it possible to duplicate/mirror screen connected to different graphic cards?
I would assume this restriction is because the OS is relying on the graphics cards to just duplicate the signal (aka Clone Mode). Otherwise either the driver or the OS would have to copy the buffer from one card to the other (and their could be lag in the output between the two screens). Duplicating the signal is a relatively easy thing to do on a single card (there are Y-adapters that do the same thing).
For extended display it sending a different set of the screen to each card to draw.