I understand that you don't have a robot and all, but here is how I, personally, would decide upon a driver/operator. Team RAGE does not choose drivers the same way I would, but I think my way would work better

.
First, I would test for
common sense: Does the driver learn from specific mistakes made while practicing? Or does s/he keep repeating the same mistake? This sounds trivial to some, but look at it this way. If the driver doesn't learn from his/her mistake now, then when in a match and s/he tries something that doesn't work, s/he will either keep trying the failed move or sit there not knowing what to do. I do understand that it is a responsibility of the coach to tell the driver what to do, but I say that this weight
should be placed on the driver, allowing the coach to worry about other things.
Next, I would test for
quick reflexes: Does the driver respond quickly to changes during a match? Does the driver respond with the
proper solution in the wake of changes that can adversely affect his/her alliance's chance of winning a match? Are the responses executed in such a way that the robots on the driver's alliance are placed in as little danger as possible? Yea-- its important.
Next, I would test for the ability to
follow instructions: Does the driver follow the instructions given by the coach? Is the driver able to, if not able to follow the exact directions given by a coach, reach an acceptable compromise? Somewhat important.
Finally, I would test for the ability to
give instructions: Can the driver effectively communicate to his/her partner what s/he is trying to do? By that I mean, are step-by-step instructions given, or does the driver just put across the general idea behind his actions. The latter is the preferred method, in my opinion. Very important.
Skill does factor in, but if the driver is good at all of the above, then refined skill skill can come later with alot of practice. I think the same thing of knowing the rules.
Oh, one other thing: Be certain that your two drivers work well together. If they don't have "chemistry," then despite their skill, coordinating movements to make them fluid will be very difficult.