The advise given by medical staff to a couple where the two partners are positif, is to continu with safe sex, because of the possibility of becoming infected by new, resistant virus strains.

Is it possible that renfection reflects only in increasing of viral load (without finding resistant strains)? Is it seen that he viral load set point changes?

I'm not sure I fully understand your question. There is considerable debate among experts and others about how likely reinfection with HIV is between two people who are both HIV+. Once someone is infected with HIV and has antibodies, it is probably very difficult for another virus to establish infection. However, many feel that it is possible and want to avoid it because infection with another strain can be problematic, whether it is resistant virus or not. Generally, transmission of HIV is more likely with higher viral loads, however, HIV can still be transmitted when plasma viral load is undetectable. Finally, anyone living with HIV should be concerned about other STDs and try to protect themselves from them. Bottom line is you must weigh the odds and decide if the risks you take are worth it to you.