Dongles to connect 2 monitors to a 13" mid-2010 MacBook Pro (mini DisplayPort)? I won't be using the integrated display. Right now, one external monitor is connected through a mini DisplayPort to VGA adaptor. How should I connect a second?
Another option to to use a display multiplexer, like the Matrox DualHead2Go DP.  This will have much better performance (frame rate / snappiness) than the USB solution, at about 4X the price.  It's best with matched dual external displays - or triple; there's also a 3-monitor solution.  I haven't used one personally, but have recommended it to clients with success.  The reviews are quite good.