How can I use two 30" LCD screens with my 2011 MacBook Pro?
The problem with the USB2VGA is that you won't be able to drive the full resolution of a 30 in, which is 2560x1600.
Technically, the 2011 MB Pro is capable to drive the 2 monitors, but since there is only one thunderbolt connector you can only connect one Mini Display Monitor. You can however connect two Apple Thunderbolt displays, since they connect to the MB Pro using Thunderbolt and not Display Port. Then you would have the use of the full resolution of both displays.
Sorry, but a 30 in monitor is the biggest one that can be connected to any computer, adding one is asking a lot from a connectors-limited portable...
Connect your first LCD with the miniDisplayPort to VGA/DVI converter. And connect your second LCD with a USB2VGA cable.