Hi, I have the 2011 iMac 21.5", which has one thunderbold port (not two like on the 27" iMac) and I would like to connect two monitors to it. I already have one DELL monitor connected to it. It works great. How do I connect the second monitor? I do not want to use Matrox. Is the USB to DVI my only option? Important: I need all three monitors to be independent from each other - i.e., to display different things. Thanks.