I am having the same problem exactly. However my main display is via the Thunderbolt port, the second is via a GUC2020DW6, and since loading the latest DIsplaylink driver I am having Display issues eg. Boot up and there is no login on the two displays. (open the MBP cover adds a 3rd display, but still not login prompt anywhere that is visible, but it is somewhere. Displays are all perfectly "grey". The only way out is to power off.
I have also noticed, with no other changes except adding the new displaylink driver, console has multiple "Displaylink Crash" diagnostic reports, and often the system hangs and will not shut down. Have to power off.
This is really a problem because it can do filesystem damage. Twice I have had to boot from USB from lion to get to the disk utility and run a clean of the filesystem on boot drive unmounted.
I wouldn't even mind spending another $150 for two newer USB to DVI converters if I knew they worked under Mac OS X 10.8 Mountain Lion....
(Checked I/O Gears site, and they list a 1.7 driver only for up to 10.6! Talk about being behind!! Two full Mac OS X revisions. Clearly there are issues with 10.8 and the driver for the GUC2020DW6. As I said, I will take a solution recommendation that requires I buy another pair that runs on 10.8..
Thanks