Fact: Fact: When two HIV-positive guys with different virus subtype, having unsafe sex when viral load is still detectable, there will be a chance their HIV viruses will infect each other, this is called HIV superinfection. The new strain of HIV can replace the original strain or they may mix and produce a new mutant. For some people, superinfection may cause them to get sick faster because they become infected with a new strain of the virus that is resistant to the medicines they are currently taking to treat their original HIV infection.
Most people living with HIV can achieve an undetectable viral load by taking antiretroviral therapy, typically within around 6 months. Once their viral load becomes undetectable, they will not be able to transmit the virus to their sexual partners.
However, even if the viral loads of both guys are undetectable, if anyone of them have unsafe sex (does not use condom) , they could still get STIs, such as gonorrhea, syphilis, hepatitis B and C.