Note that tray tools is a little buggy. I still haven't found a way to change the active GPU for overclocking settings. There is a checkbox to make all changes to both GPUs. What I do is set both cards to 940/331, then I uncheck the box and set the rate to 840/250. This leaves the second GPU at 940/331 and changes the first GPU to 840/250. Also, each time you change the clock rate it sets the fan speed back to automatic, you'll have to go change that back to 100 percent.
Got it, it's working. Thanks so much, I spent all day trying to troubleshoot this. +karma!