Reply to your reply:
I believe you would want to control all the variables, but that's not the same as having a control experiment.
For example, if you were testing 2 enzymes that break down Hydrogen Peroxide, you'd want to mix 1 batch of hydrogen peroxide in water, and distribute that between two testing chambers, then add equal amounts of the two enzymes and run the experiments simultaneously so that environmental factors (light, etc) did not play a role. Having a control of just hydrogen peroxide water with no enzyme is optional, but not required, if you're only showing a comparison between the two enzymes. If you were trying to prove that the two enzymes were, for example, THE SAME enzyme, you WOULD want a control with no enzymes, and perhaps an additional test with a third enzyme, to allow you to compare the 4 reaction rates.