Despite the established commercial polymer composites available for producing bipolar plates for fuel cell stacks with suitable conductive properties, there are still research activities necessary to develop bipolar plates with maximized electrical and thermal conductivity. The common pathway to develop such materials is to enhance the content of conductive fillers (e.g. graphite) in the composite. Other research activities focus on the use of expanded graphites that have high intrinsic electrical and thermal conductivity. The approach of using synergistic effects of bimodal filler systems based on two graphites of different types and particle-size distributions is not deeply explored in literature. There is a need to develop recipes with maximized conductive behavior by holding down the proportion of conductive filler to enhance the mechanical properties and plastics processing. In this study, composites based on polypropylene (PP) and different filler systems are melt-mixed using a small scale microcompounder as well as a lab scale co-rotating twin-screw extruder. The measurements of the electrical and thermal conductivity as well as mechanical properties of the composites are performed on pressed plates. The influence of synthetic/synthetic and synthetic/expanded graphite combinations and their different mass ratios in the composite on the conductive and mechanical behavior are investigated. The results show that a composite with 80 wt% of a bimodal filler system of two synthetic graphites (mass ratio: 4:1) and 30 wt% ethylene-propylene-diene-rubber-monomer in the PP component has a lower volume resistivity (0.17 Ω·cm) than the most electrical conductive composite with monomodal synthetic graphite (0.31 Ω·cm) at the same thermal conductivity of 15.1 W/(m·K). Additionally, this combination shows a lower volume resistivity and same thermal conductivity as compared to a composite containing 80 wt% of highly structured expanded graphite (0.19 Ω·cm, 15.5 W/(m·K)).