I'm not sure how to implement this myself but it's certainly doable
The principal is that in a dual/multi monitor system, most of the time you don't need all those monitors powered up.
A service watching window positions would be good, when you move an application towards a second monitor position, it could power up that monitor and when a monitor is windowless it could shut down again. In this day and age of power saving, this should be built into Windows.
