Audio output selection

I’m on a G5 which has 2 line outs and 1 digital out. You can select digital out vs line out in the system preferences. I would like to be able to turn just one of the the line outs off. Anyone know of any software which would do this, and is scriptable?

EDIT: The reason for this is that I have headphones plugged into one of the line outs and speakers plugged into the other, and would like to not send a signal to the speakers most of the time, but allow an alarm clock script to turn them on.