This is the original form of efficiency. You have two compressors on one tank, when both are needed, both run, when one is needed, the other is off and not using electricity. These systems come in both reciprocating and rotary screw packages. They are great for applications that have a large air demand for a portion of the day and a smaller steady demand for the rest of the day. When the demand is large, both compressor packages can run in unison and keep the application going. When the demand dies down, only one compressor will run cutting your electrical usage in half. Another benefit is that you have two compressors instead of one so that if one compressor fails for any reason, you always have a backup. A downside to duplex compressors is that they are piped together to a common tank, so if a fitting or piece has to be replaced on the tank, both compressors packages are down. When you have two separate packages that are independent, a repair on the tank of one package will not affect the other.