If both you and your partner are hiv negative, and have had no other partners. can you have unprotected sex more than once and still get hiv?

Sure, but...: Catching HIV in this scenario would have to be from a source other than the 2 non-infected people. In the US, such an infection has about a 0% chance of happening in a person without risk factors, mathematically rounded to 0% of course. Risk factors like sharing needles would make the chance of catching HIV much higher. The virus has to come from somewhere (it does not create itself from nothing).