I have connected a 12volt battery to a sabertooth 2x25 controller that then attaches to two seperate 12v motors and to a r/c receiver. Everything works get except it moves to fast. I even put a stepdown to 5v to the Sabertooth and still moves supper fast.
Generally, you would have to either reduce the voltage to the motors or limit the range of the R/C signals send to the Sabertooth.
However, you say that even at 5V, the motors turn too quickly? What motors are you using?
Reducing the R/C signals needs to be done either with advanced features in the Remote Control, or using a device such as a microcontroller to reduce the range.