A gas burner assembly for a cooktop appliance includes a normally aspirated primary burner and a concentrically-positioned forced air boost burner. A dual-outlet control valve provides a flow of primary fuel to the primary burner through a first outlet and a flow of boost fuel to the boost burner through a second outlet. The boost burner operates by receiving the flow of boost fuel and a flow of combustion air from a forced air supply source only when the primary burner is operating in a low condition.