Sexual contact between two HIV infected people most certainly requires a condom. Different strains or types of HIV can be passed between two HIV infected people, making treatment of the infection even more difficult. This transfer of one HIV strain to another HIV infected person is called reinfection
For instance, if person "A" has an HIV type that has been responsive to therapy and person "B" has an HIV type that hasn't, passing that type of HIV from "B" to "A" will make it harder to treat person "A", possibly making therapy ineffective in person "A" as well. Also keep in mind, condom use is also important in preventing the transmission of other sexually transmitted diseases.