The question is, in what way is your drivetrain "too fast"?
Is the top speed more than your driver can handle/control? If so, either more drive practice, or gear lower as suggested above so that you will use less current accelerating and/or accelerate to top speed faster.
Is the acceleration (particularly at low speed) too great, causing you to fall over backward? If so, less grippy wheels or voltage ramping (programming so that you are only increasing the voltage at a maximum rate, e.g. 20% throttle per 100ms) may be the answer.
If the joystick is too sensitive in "parking" maneuvers (e.g. lining up to get or score game pieces), you could have a switch to go between two throttle modes, or program a nonlinear response to the joysticks.
Ether's paper on joystick sensitivity is a great resource for the last.
Edit: I've never written any LabView, so forgive me if my terms are way out of whack, but if you really still want to do what you asked at the beginning, you would add two "multiply" operators. Break the wire connecting your joysticks (or arcade operator) to your motor controllers. Feed the joystick output to one input, 0.75 to the other, and the output goes to the motor controller input.
That throttle switch idea would give 1.0 at one throttle setting and (for example) 0.5 at the other where I had 0.75 above.