Molecular doping is a powerful tool to tune the thermoelectric (TE) properties of solution-processed semiconductors. In this work, we prepared a binary composite and effectively doped both of its constituents, that is, naphthalene diimide–bithiophene copolymers (PNDI2OD-T2) and single-walled carbon nanotubes (SWCNTs), by a 1H-benzimidazole derivative (N-DMBI). The doped composites show an n-type character and an in-plane TE figure of merit (ZT), exceeding the values obtained with the doped polymers. The use of SWCNTs consistently results in a higher σ with a maximum value above 102 S/cm, resulting in the highest power factor of 18.1 μW/mK2 for an SWCNT loading of 45.5 wt %. Furthermore, an SWCNT content up to 9 wt % does not compromise the low thermal conductivity of the polymer matrices, leading to a ZT value of 0.0045. The n-type composites show good solution processability and relatively stable Seebeck coefficients upon air exposure for 8 months.