No it does not list a second monitor because to my computer I only have one monitor. And yes I have Show Full-Screen Preview set to on.
The way the TripleHead2Go works is I have one DVI cable coming out of my computer to the TH2G box. Then the box splits the signal into three separate DVI signals which goes to my three monitors on my desk. So my computer it thinks I have just one monitor that's simply HUGE. The TH2G box does all the video splitting.
Right now the only way I'm able to get an external preview is to hook up my right most monitor to the 2nd DVI port on my video card using a VGA cable and connecting that to the VGA port on the monitor. Then switch the monitor from DVI to VGA. I loose 1/3 of my desktop screen when I do the preview but it works.
What I would prefer is that each element of Studio is in it's own desktop window. This way I can move the preview window to my right monitor, move the tools to my left monitor and keep the timeline in my center monitor.