Two phase electricity supply is where you have 2 live (positive) feeds to your property and will therefore you will need 2 sensors for your energy monitor. 2 phase systems are sometimes used for multiple tariff supply such as Economy 7.
Two Phase electrical systems using 110 -120 volts are prevalent in the USA, Canada, Central and some parts of South America, the Caribbean, and parts of the Pacific. As the cables used in the USA and Canada are larger than those commonly used elsewhere in the world, larger sensors are also required. so we have a separate sub category for energy monitors for use in the USA and Canada