you quantify this by disabling v-sync, seeing you get 99% GPU usage, AND more performance.
You're lucky that the titles you enjoy have support. most don't BF4 will allow dual card, but I'm pretty sure it requires a little bit of work. Fortunately, Frostbite has baked in DX11 mGPU, it just has to be force enabled.
Most other engines aren't quite as friendly, or will give you "fake" SLI ,which is easily proven/disproven by no-vsync gpu usage and perf #s.
ORRRR if the SLI does work, the stutter it injects negates any high frame benefits as you'll go from an epic 120FPS down to 40, back to 120 so quick that all you see is a giant DERP.
my last good, functional SLI setup used that same architecture
. 3x OG Titans. 99% of the time, card #3 is off, even in sli supporting titles.