Monitor Tests Forum

From what I've read, whenever testing for hz rating or skipped frames, you can only have one monitor connected/displayed. This is the case for me also when using http://www.testufo.com/#test=framerates

If I have two monitors connected, it halves the frame rates. When I run a full screen 3d program, my frames are not halved.

Though how do I know I am still achieving 120hz? Does the multi-monitor setup limit this in actual games?

That's more of a browser limitation. Multiple monitors will not affect the refresh rate, but the frame rate might be affected with vsync on because it might only synchronize with one monitor. If it tries to synchronize with both, there's no guarantee that both monitors will refresh at the same time, which complicates frame timing, especially with mixed refresh rates.