As Gray05 wrote:
"Pull through that first stage while the planets align and just break that icicle when your sight picture is right.
A single stage would either have no travel and break at the slightest touch or have a very smooth travel."
So, isn't the key here to minimize movement in the firing process as much as possible, keep those crosshairs still, watch your breath, heart-rate, etc? If so, then we'd want as little finger movement and as little muscle contraction as possible, no?
2 stage - you are slightly remaining in a active muscular contraction all the way through the first stage until the trigger breaks at the 2nd. For precision work, that becomes more motion to control when trying to stay on target.
Single - you'd need to move your whole finger from the straight position, onto the trigger, then only press ever so slightly. Now, there would be movement, but less contraction. (I know, you'd better have a real good safety if you're using a hairpin single stage for precision work!)
I guess they both have their benefits and draw-backs, and of course it will always come back down to whatever a person shoots best with (given all other factors are even)
-obviously, I'm just thinking online / rambling...
DRD