You're not taking shaft displacement into account. When you do that it's a wash.Dampening is what happens to a lady's underwear when they see a sweet prerunner, as in "hey Brit, that sweet prerunner is dampening my panties". Damping is what shocks do, as in "hey Brit, that prerunner has some buttery smooth damping".
Single 2.5" - 2.5^2*pi/4 = 4.9in^2
Dual 2.0" - 2*2^2*pi/4 = 6.3in^2
So for the same travel of shock, dual 2.0s will provide more oil and in theory would run cooler, but dialing two shocks is harder than one. Plus most 2.5" shocks have larger resis, so the larger cross-sectional area is likely moot. The first truck I ever built ended up running a single 2.5 King with SAW torsions before being sold and it was buttery, though that was before I had built more advanced trucks.