I am not sure what your sources are. There is a lot of mythology regarding any variety of things modifying seroconversion including antibiotics, other infections, etc. As you might imagine, given the rarity of documented needle stick seroconversions to HIV in situations where such things can be measured, the chances of having a meaningful study of such issues is an impossibility. Thus we need to deal with what we know about each disease. There is no scientific reason that infection with two pathogens like HIV or hepatitis C should interact to delay seroconversion.
My advice to you is that if you are now negative for both infections now, at 7 weeks post exposure, it is most unlikely that you acquired infection and that with each passing week your probability of infection declines further. Hope this helps. EWH