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You could either use a solution from Matrox (DualHead2Go) or a USB-to-DVI adapter, which often use DisplayLink technology. The first solution is probably the "smoothest" (for video etc), but the 2 screens need to have the same resolution. The second solution doesn't have this limitation, is probably cheaper (I got one for about 40 euros on ebay), is hot plug-able (once the drivers are installed), but is not meant for video playback (A youtube video works, but don't expect fullscreen videos).
 
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