Currently, there are no specific recommendations regarding the management of the synchronous tumours due to the lack of either specific guidelines or individuals' clinical experiences relative to these clinical situations. In the presence of a locally advanced double primary tumour and with the lymph node metastases in addition, from the radiotherapeutical point of view, it must be challenging to manage this complicated situation that requires a more delicate treatment planning, due to higher doses prescribed to greater volumes concomitantly with the chemotherapy. A 68-year-old Caucasian male with a synchronous intermediate-risk prostate adenocarcinoma and locally advanced anal canal carcinoma underwent IMRT-SIB with concomitant chemotherapy at our institute. Two years after the treatment, the restaging CT and MRI scan showed no evidence of the disease and the patient reported no significant gastrointestinal or genitourinary toxicity. Our experience is unique, since it is the first report on using the IMRT-SIB technique simultaneously with chemotherapy in the management of the synchronous prostate and anal canal carcinomas. Therefore, we find it important to provide the current literature with the results from our experience which show good feasibility, efficacy, and tolerability of the definitive concomitant IMRT-SIB-chemotherapy for the synchronous anal canal cancer and prostate cancer.