Display omitted •A bimodal composite foam with CNT-GNP hybrids was prepared.•CNT and GNP show limited effect in improving electrical properties of composites.•CNT and GNP synergistically enhance electrical properties of composite foams.•The synergistic mechanism of CNT and GNP in composite foams is clarified.•The bimodal composite foams can be used in EMI shielding. Novel PMMA composites were fabricated using two different dimensional nanofillers of MWCNTs and GNPs as bifillers to explore their synergistic effect on the final mechanical and electrical properties. MWCNTs and GNPs display limited effect on the final electrical properties while they show obvious synergistic effect in influencing the mechanical performance. The well-dispersed MWCNTs-GNPs in polymer matrix provide plenty of 1D-2D interconnections for efficient stress transferring when exposed to stress tensile, but the critical distance between nanofiller is not enough for effective electron movements and the significant Schottky barrier between nanofiller contacts has restricted the synergistic effect for electrical properties. Afterwards, PMMA composite foams with microcellular structure were prepared by supercritical fluid assisted processing. The ternary bimodal composite foams present better tensile, electrical and EMI shielding properties than the unimodal composite foams. MWCNTs and GNPs reveal prominent synergistic effect in influencing the tensile and electrical properties of ternary composite foam systems. The obvious enhancement of electrical conductivity results from the in situ exfoliation of GNPs, high-level orientation and redistribution of nanofillers, moderate physical foaming with bimodal microcellular structure as well as attenuated Schottky barriers at few-layer graphene/MWCNTs junctions. The dramatic improvement of EMI shielding performance is attributed to the elevated electrical conductivity, unique foam microstructures and special 1D-2D (MWCNTs-GNPs) hybrid structure. This study paves a feasible road to prepare lightweight and novel composites with superior electrical properties.