This is repeatedly used as an example in the medical community about the limits of randomized controlled trials. This isn't some impression - your impression that such evidence exists is wrong.
There might be some parachute company tests about effective of velocity, etc., but there are no human trials.
Why? Because that would be unethical.
It's a good thing "randomized controlled trials" aren't the only kind of empirical evidence, then.
We know the limits of how fast a human can safely land. Parachute manufactures have to prove that their designs meet the minimum performance specifications to achieve a safe speed. This proof is not invalidated by the fact that it doesn't include throwing some poor bastard with a placebo parachute out of an airplane to demonstrate that he dies on impact.
Also, the answer to your original question is yes. There are numerous studies showing that multiple monitors improve productivity.
Are you under the impression that we don't bother to empirically prove things that seem obvious, like the safety benefits of parachutes? You don't think parachute manufacturers test their designs and quantify their performance?