I have just solved this.
I have two 24" Samsung S24B150 monitors that support both VGA and DVI.
I attached one monitor via a mini display/thunderbolt adapter from Apple and the second on to a J5Create JUA230 USB to DVI converter (there are also models in the line that support HDMI and VGA). FYI this is not a USB 3.0 device, but I've not had any performance issues.
The process is simple.
1. Download and install the latest driver from the j5Create site. (It does come with a DVD, but they suggest you download the latest driver from their site www.j5create.com)
2. Restart your computer (this is required after the driver is installed)
3. After the computer is rebooted (log in if required) plug your monitor into the adapter FIRST and then plug the adapter into your USB port.
4. The second monitor popup right up me (you may need to go into Preferences/Display, but I didn't)
This is a $65 solution, but after trying several $200+ docking station that didn't work, I purchased a very nice Anker USB hub (3 high powered and 7 regular USB 3 ports) for $40 and the j5Create adapter and for about half the price had everything I was looking.
If you don't need the Ethernet and audio abilities, which I didn't, this is a great solution. And it WORKS!