Hey guys, so there's a little backstory on how I ended up here. I currently have a pair of GTX 770s in SLI. The top card is a 770 SC (with ACX Cooler), the bottom card is a 770 Dual SC. I'd have liked to have avoided this (running different cards) but the availability on the cards when I got them prevented me from doing so.
 
I've had a lot of difficulty getting this to push past stock clocks. Valley would crash on scene 6 everytime I moved the GPU Clock even slightly. I asked for help on another forum, there was a guy who had some decent overclocks on his SLI'd 770s. He recommended using Precision X16 (I was using an older version waiting for X16s bugs to be fixed), and using overboost instead of overvoltage. I figured this would work.
 
Valley didn't crash, but the framerates tanked something horrid. Here's the chart from Precision

 
 
Upon further inspection I realized that the second card wasn't accepting the overboost, and I suspect this was part of the problem (With it still being on stock voltage). My question is why - and what can I do to get around this?