You always need two Group Addresses in order to link a sensor with an actuator.

False. A single Group Address is enough to link a sensor's sending Group Object with the corresponding receiving Group Object of an actuator; both Group Objects subscribed to the same Group Address will exchange the value (e.g. ON/OFF). A second Group Address is only added when status feedback is required, so that the actuator reports its state on a separate address that the sensor (or a visualisation) can read back, but this is an optional design choice, not a requirement.

For the simplest 'switch a lamp' case, use one Group Address; reserve a second 'status' Group Address only when the project needs visualisation feedback or scene synchronisation.