Yes it's because of HCV ability to be survive outside the cell longer and this time for HIV is short.
I don’t understand your question. Both diseases are not too common (at least not where I am from) and the combination is rare as HCV is now treatable (again, where I am from it is available for all patients). Also, when treated HIV has a low virulence too. So, do you mean incidence under patients, which of the two has a higher virulence or do you mean something else?
Is it because the virus is able to survive outside the body longer? What is the reason for this? Thank you by the way for this post, I did not know this!