well since stacking caps at 133%...it will take more time if you stack them then alternate between skills.

However there are reasons for stacking (if the skills you are using are only 25% or less)

A. It gets you out of a tough bind quicker
B. When running shorter distances, you get there quicker


When running a clear path, if you were to use both skills at once instead of one and then the other, there wouldnt be much difference as the average time would be the same (without stacking :P )