I'd imagine that the answer is a simple one: not everyone has two (or more) monitors.
A more complicated answer is: The testing environment must be configured the same for all test-takers. Otherwise, disparity would be introduced, compromising the integrity of the tests.
All of my RH tests, which I've taken at a testing sites, as been with one monitor.
Although I have no access to a testing environment, I do have access to the lab environment for the RHCSA (RH124 / RH134) and RHCE (RH294) courses (I'm an instructor). I do not know for certain that the lab environment is the same as the testing environment -- but they should be the same. Why would Red Hat have you train in one environment and then be tested in something different?
Anyway, none of the VMs have mutiple video adapters (or displays) installed.
That's my 2c
thanks Tracy. What I found most annoying was I found the test environment booted to from a usb stick was slooooooooow to respond. This made having one monitor very frustrating as I'd be clicking the icon to open another app, yet the o/s would not recognise and act on this immediately. I can't remember the exact lag between clicking and doing, but it was a quite noticeable one, at least a second or two. I'd click something I wanted to open, it wouldn't immediately respond like a real world, real PC scenario, so I'd either click it again, or click something else. Half the time I was catching up with what my mouse was doing, or vice versa.
The most frustrating part of answering the questions is not being easily able to refer back to the question while you're answering it.
Allowing , (and the vm having), a second monitor would allow test takers to move the screen with the question to a second screen for reference, whilst answering it on the other.