These two signals are not independent. The 0 - 10 volt signal is derived from the PWM signal internal to the X-controller. The only way to change the value of the 0 - 10 volt signal is to manipulate the PWM signal.
Since both the spindle and laser are controlled by the PWM signal you should set something up external to the X-controller to select only the spindle or the laser when you have both attached to the X-carve.
The most simple way to do this is to only apply power to one device at a time. This is not limited to the X-controller. Using the Arduino/gShield should follow the same advice.