think iv come up with a wiring diagram that might work for you. ( someone please check as im rather sleep deprived)
have had to assume a few things though, namely :
- the pwm modual requires a + and -ve and outputs a seperate pwm +ve
- the pwm modual dosn't mind having 24v applied to its output
- the joysticks have a neutral point in the middle where none of the contacts are closed
if no's 1 or 2 are wrong we should be able to make minor changes and still be good, if no 3 is wrong we will have to come up with something else.
whoops, that will go bang

should work as follows:
S joystick moved left, power is applied to PWM modual and R1, pwm signal applied to motor, motor moves left
S joystick moves right, power is applied to PWM modual and R2, pwm signal applied to motor, motor moves right
F joystick moves left, 24v is applied to motor, motor moves fast left
F joystick moves right, 24v is applied to motor, motor moves fast right
iv added interlocks to stop both R1 + R2 ( left and right) being energised at the same time
also if the motor is moving under pwm control and the fast joystick is moved in any direction, it will override the slow speed and move at full speed.
hopefully thats all correct and what you were after, if not it should be a basis to work from.