Advanced coolants such as nanofluids or PCM slurries are realized and reported as effective substitutions for conventional coolants in order to enhance the cooling performance of microchannel heatsinks. However, there are a number of disadvantages associated with using these advanced coolants that are always considered as their inevitable downsides including the increase of the needed pumping power of the system for both of the mentioned coolants and decrease of the fluid average thermal conductivity for PCM slurries. In this paper, a 3D conjugated heat transfer model of a double layer microchannel heatsink (MCHS) to investigate its thermal performance is presented and the effectiveness of simultaneous employing of both of these types of advanced coolants in a heatsink is assessed. Finite volume method (FVM) is used to solve the flow and heat governing equations simultaneously and the obtained results are validated with the experimental data. The assessed parameters in this study are: coolant type, coolant configurations, particle concentration, bottom surface temperature, and inlet fluid velocity. In order to investigate the flow and cooling efficiency of the heatsink, two dimensionless parameter, Nusselt and Euler numbers, are computed and compared between the considered configurations. The goal is to balance and optimize the desirable cooling enhancement that is obtained by using advanced coolants and is represented by associated Nusselt number from one side, and undesirable increase of the needed pumping power which is implied by relevant Euler number, from the other side. Results showed that using the proposed configurations, the cooling performance of the systems are enhanced and the disadvantages associated with advanced coolant are relieved, substantially. (C) 2015 Elsevier Ltd. All rights reserved.