Just to note that I did eventually try this and I could not get the TH2Go to work at full resolution but I think that is because I connected it to a HDMI port rather than DUAL-DVI. I might try it again in the future. The monitors did show up and worked well, just at a lower resolution. I'm pretty sure it was just that I had hit the bandwidth limit of the HDMI cable/port.
However I did manage to add 2 accessory monitors to my nvidia surround setup by having one as an nvidia accessory and the other (the fourth) connected to the motherboard's internal INTEL HD GRAPHICS option. Once the drivers were installed and the onboard video enabled in the bios the extra monitor worked fine at full resolution.
So it can be done...
I have an ASUS Z97 PRO and GTX970.
Many new motherboards have inbuilt intel video so you might be able to get at least 5 displays on one computer without spending any money. Also the Z97 PRO has a multimonitor setting which I enabled so maybe both the VGA and DVI motherboard connectors will take monitors. I'll give that a go too. That would equal 6 monitors