If you set the unused zone as ‘parallel’ — in other words, turn off it’s ‘sequential’ flag, then you can send command to it to turn it on or off at any time. By default, all zones have the ‘sequential’ flag on. With the sequential flag on, if there is any other zone running (or scheduled to run), it will be serialized (which is probably ok because it will run eventually when the other zones finish running). However, if you don’t want it to be serialized with other zones, turning the sequential flag off would do it.