Two technologies are currently competing for providing Vehicle-to-Everything (V2X) communication services in Europe and in the 5.9 GHz ITS band. The oldest one is known as ITS-G5 and specified by ETSI, and the new comer is standardized in 3GPP and derived from LTE technology. It is now conjectured that both technologies will coexist in the deployment phase, at least during a transition period. Several coexistence scenarios are currently under study. Some of them are even considering the option to share a 10 MHz radio channel in the 5.9 GHz band among both technologies under the same geographical area. This approach is called co-channel coexistence. New mechanisms are currently explored and assume some modifications of the standards aimed at minimizing the performance losses. This co-channel coexistence scenario requires a careful assessment, and this article proposes the first performance evaluation in this particular context of coexistence between ITS-G5 and LTE-V2X standards. It presents a simulation work that has been carried out with the purpose of assessing performance of mixed deployments, and also assessing the efficiency of one possible coexistence mechanism. It is based on system level simulation which is itself relying on physical layer performance assessment pertaining to both technologies. The simulation methodology is presented and evaluation results are provided and commented.