When I play games in fullscreen on the BenQ, the Samsung will either turn black, freeze frame, or work perfectly. There seems to be about the same chance of either of the three happening each time I alt-tab in or out of a fullscreen game. I would chalk this up to a limitation somewhere in the hardware if it didn't actually work about 1/3 the time.
When I play games in windowed, or borderless-fullscreen, I get functionality of the Samsung, but I fail to get 120hz gameplay. The framerates are more choppy and look like they are at 60hz. If I switch to fullscreen, I get the 120hz goodness, but no functionality on my second display.
The BenQ is installed correctly and is running at 120hz.
The Samsung is running on Generic drivers at 60hz.