AIDS is a virus, it can't be created in a sexual relationship. It needs to be transmitted, so if both are negative, there's no way they can get AIDS, but it's hard to tell with 100% accuracy that someone is negative.
And the same is true with any contagion method, vaginal sex, blood transfusion, sharing needles, even a mother giving birth to her son can transmit AIDS to the child, but the mother has to be positive... there's no sense in a child getting aids from an HIV negative mother... and the other way makes even less sense.