The 717-140A-DVSR is designed for twin engine installations. It allows the Port engine to charge both the Port start battery and the house battery when the Port vsr is engaged. It also allows the starboard engine to charge both the starboard start battery and the house battery when the starboard vsr is engaged. This gives you a combined charge from two engines into the house battery until the voltage regulators on both engines control the charge. It also allows you to charge the house battery if steaming on one engine.