I had bladder cancer five years ago, probably caused by the radiotherapy on my prostate five years earlier. I had repeated treatments before I got the all clear on the BC. Because of the recurrences and the high risk of a further recurrence and the fact that BC is much more dangerous than PCa, I was advised to go for the ultimate treatment of bladder and prostate removal. I did so and the BC has never returned. Whilst I hope you are never faced with such a decision, I want to assure you that it is not the end of the world to have that radical cystectomy and normal life goes on.
My best advice is to focus on getting rid of the BC. You can take your time with the other horror show. To the best of my knowledge although radiation of the prostate is known to carry the risk of setting off bladder cancer, the reverse is not true, ie, bladder radiotherapy has not been found to cause or exacerbate prostate cancer.