In a first aspect, there is a rotor system for a tiltrotor aircraft, the rotor system including an upper outboard engine in a fixed location on a wing member of the tiltrotor aircraft; a lower outboard engine in a fixed location on the wing member; and a prop-rotor pylon in power communication with the upper and lower outboard engines, the prop-rotor pylon being configured to selectively rotate between a vertical position and a horizontal position. In another aspect, there is provided a tiltrotor aircraft including a fuselage; a wing member; an upper outboard engine in a fixed location on the wing member; a lower outboard engine in a fixed location on the wing member; and a prop-rotor pylon in power communication with the upper and lower outboard engines, the prop-rotor pylon being configured to selectively rotate between a vertical position and a horizontal position.