A fluid immersion cooling system includes a fluid tank that contains a layer of a dual-phase coolant fluid and one or more layers of single-phase coolant fluids. The dual-phase and single-phase coolant fluids are immiscible, with the dual-phase coolant fluid having a lower boiling point and higher density than a single-phase coolant fluid. A substrate of an electronic system is submerged in the tank such that high heat-generating components are immersed at least in the layer of the dual-phase coolant fluid. Heat from the components is dissipated to the dual-phase coolant fluid to generate vapor bubbles of the dual-phase coolant fluid. The vapor bubbles rise to a layer of a single-phase coolant fluid that is above the layer of the dual-phase coolant fluid. The vapor bubbles condense to droplets of the dual-phase coolant fluid. The droplets fall down into the layer of the dual-phase coolant fluid.