Wire the "Forward/Reverse" pot with R2, 10kΩ to ground and R1,39kΩ (instead of 100kΩ) to the plus voltage as shown on the schematic but connect its wiper to the wiper of the "Turn" pot. Connect one end of the "Turn" pot to one of the controller inputs IC1:A-3 and the other end of the pot to the other controller IC1:A-3 input. Also connect resistors from each of the IC1:A-3 inputs to ground. (Try 100kΩ to start, but you may need to experiment with the values to get the desired control function).
That should more or less give you the control functions you want. The "Forward/Reverse" pot will control the speed and direction of both motors. The "Turn" pot will vary the relative speed between the two motors.