Do you think it is possible to take 2 20 mg Cialis in one go ?

I've tried Cialis at five different times. I'm approximately 80% erect and after which I "hit the wall". This is all it takes. After the 80 percent sexual erection, it becomes very much flaccid. Following the 80% erection, ALL is gone. I'm overweight and am in my mid-60's. I was treated with radiation to treat prostate cancer in the past. What's the next step ? Can you use two 20 mg tablets in one go? It is WAY expensive for NOTHING !!!