Dimmer doubling allows for remodeling and upgrading older systems, adding double the lights without needing twice as many dimmers and cables. ETC refers to the system as dimmer doubling, but most theater techs call it multiplexing.
The dimmer doubling system can be a benefit in saving money and space. The cost savings is in fewer wires and fewer dimmers being required to run the system. This can also be key for lighting trucks on portable systems with limited board space. Also, you have a greater variety of lighting options with the same number of dimmers.
The dimmers require a special 77 volt lamp to operate. Using the lower voltage lamps is the only way to get the full light intensity with half of an AC wave. Dimmer doubling does not save electricity, because the total output of the two lamps can only equal the maximum load of the dimmer. So the total output of the lights is the same for the same cost of electricity.