An air conditioner and a method for controlling an air conditioner are provided. The air conditioner may include at least one indoor device; an electric heat pump (EHP) outdoor device connected to the at least one indoor device, configured to drive a first compressor using electric power, and having a first outdoor heat exchanger for heat exchange of a refrigerant; a gas heat pump (GHP) outdoor device connected to the at least one indoor device, and having an engine configured to drive a second compressor using a burned gas, a second outdoor heat exchanger for heat exchange of the refrigerant, and a waste heat exchanger configured to exchange heat with the refrigerant using waste heat of the engine; and a low pressure refrigerant common pipe configured to connect the first outdoor heat exchanger with the second outdoor heat exchanger and to supply a low pressure refrigerant of the GHP outdoor device to the EHP outdoor device.