Can the same radiation therapy regimen treat two different types of cancer at the same time?

Yes. Unlike chemotherapy that has different drugs for different cancers, radiation is ionizing xrays that disrupt the dna. Therefore it is not cancer specific. It however has different sensitivies because each type of cancer behaves and responds differently and that is why we have different doses and techniques to use based on the cancer.
Yes. The organs must be in the same region since radiation is a local treatment. For example i had a patient with intermediate risk prostate cancer and anal cancer that received radiation to the pelvis and prostate for about 5.5 weeks ( overlapping regions that needed to be treated) followed by cyberknife boost to complete prostate treatment.
Yes. Occasionally, we see two different types of cancer within the same region e.g. Bladder and prostate cancer, rectum and prostate cancer and they may be treated simultaneously with the same radiation regime. However, the dose fractionation and overall dose will depend on the intent of the treatment and patient's condition.