I know your post is a few weeks old but I just joined the forum today.
I want to add an additional answer to the ESP Support reply. I agree with all he said but I just went through trying to further regulate the speed of the 500W motor on the kart I just built for my son.
You can wire in a potentiometer on the throttle signal wire. I've recently done this. Works really well. I've read on some other forums that you can't do that, but I can tell you it definitely works. Mine is a hall effect foot pedal. Just a simple little variable resistance piece you can get at radio shack for $2. Get the 10ohm. You turn the dial one way and it limits the power coming down the line = slowing or speeding up the kart.
The other option is getting a controller that has one built in. I got one of those from our host here. We won't discuss why I have had to buy multiple controllers though. :-) I made a little mistake but not related to the potentiometer though.
A 24 Volt 500 Watt controller can handle a 500 Watt load continuously however will output a 720 Watt load intermittently for extra power to get up short inclines and accelerate from a dead stop. As long as the two motors connected to a 500 Watt controller were geared and used in a manner where they do not exceed 500 Watts of continuous power draw then the 500 Watt controller would not overheat and be reliable.
Keep in mind that when overvolting a 16.8 Volt motor to 24 Volts its RPM will increase by around 40% so the gear ratio may need to lowered to keep the motor running at a high RPM so it does not overheat itself, or draw too much current from the controller and overheat it.