CO2-assisted co-gasification of binary mixtures of pinewood pellets (PWP) and two kinds of plastics polyethylene-terephthalate (PET) and high-density polyethylene (HDPE) were examined at 800 °C using a fixed bed reactor. Evolutionary behavior and yields of CO, H2, and CmHn were investigated for both individual feedstock and binary mixtures of biomass and plastic. Synergetic effects in co-gasification of mixtures under CO2 atmosphere were analyzed and compared between experimental and calculated results. The results showed that PET and HDPE although had similar behavior in gasification, they provided many different characteristics on blending with solid biomass in CO2-assisted co-gasification. Both PWP–PET mixture and PWP–HDPE mixture showed positive effects on hydrocarbons yield and negative effects on solid yield. For PWP–PET mixture, H2 yield showed no change compared to the calculated value; however, CO yield and CO2 consumption showed negative effects due to the blocked porosity of solid biomass from the softened PET. For PWP–HDPE mixture, H2 yield showed significant enhancement compared to the calculated value, and CO yield showed slight enhancement but a slight reduction in CO2 consumption. It was also observed that the experimental CmHn yields obtained from biomass-plastics mixtures were of higher values than the calculated values. The morphologies of solid residues for PWP, PET, PWP–PET, and PWP–HDPE were analyzed and taken as a supplement to explain the synergetic effects in the co-gasification process. These results provide an insight into energy recovery and waste treatment potential for both biomass and waste plastic using thermochemical conversion.