Ok, after some fidling I found that I can trick the x2vga to output with the correct refresh rate (85Hz) by booting into windows with the monitor plugged directly into the computer then switching the monitor off, plugging it into the vga-out of the x2vga and connecting the vga-in to my video card. When I switch the monitor on again the desktop refresh rate has been retained.
So its only when I boot into windows using the pass-through configuration of the x2vga that I cannot obtain the correct refresh rate. What exactly is the problem here?