How does the heart function as a double pump?
Because it pumps the blood outside the body twice. Once during the pulmonary circuit, and the other during the systemic circuit.
The pulmonary circuit involves pumping out blood from right ventricle to lungs through pulmonary arteries and its return to the left atrium through pulmonary veins.
The systemic circuit involves pumping out blood from left ventricle through aorta to body parts and its return through vena cavae (inferior and superior) to right atrium.
Here, blood entered the heart twice and left it twice.