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My desktop has 2 HD 6450 graphic cards, each of them has one HDMI, one DVI and one VGA port. I connected 2 of my screens to the 2 HDMI ports on 2 cards, and I'd like to duplicate my 2 screens.

However, Windows doesn't let me do it, as there's no option to:

enter image description here

I can only extend the displays, or show the desktop on one of them, but cannot duplicate them!

Even if I connect one screen to the DVI port on one card and one screen to the HDMI port on the other card, it still won't let me duplicate.

If I connect them to different ports on one card only then it works fine - I can duplicate the screen.

Is there a way to duplicate the screens if they are connected to different graphic cards?

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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.

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Further more, there's no reason to put them on separate cards if you're just going to clone them. It doesn't increase the load on the card's GPU, since it's still just generating one signal and sending it out both ports. If you want to actually use both graphics cards, connect both monitors to the same graphics card and enable crossfire. – Darth Android May 16 '13 at 19:15

The simplest solution for you is to just buy one DVI to HDMI adapter (~5$ on amazon) and plug into one video card. As mentioned by Brad, mirroring is an easy thing for a single video card to do but not 2 video cards.

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