HIV and Multiple Sclerosis treatment question?

The second week of July I was diagnosed with Multiple Sclerosis. The third week of July my husband was diagnosis with HIV. I have been tested twice for HIV and they both came back negative and I will be testing again in October because that will be the three month mark of when we last had unprotected sex.

My questions is this. I was put on Avonex to treat the MS and I was told that if I end up being positive for HIV that I will have to choose which treatment to take because the meds will counteract each other and not do me any good. Is this true?

My husband was put on Atripla and so that MAY be what I will take too. I dont find any research on MS and HIV treatments going together or counteracting each other so I was hoping someone may have an idea or know of somewhere I can find answers.

Oh, besides having to choose because of the prices of the meds (Avonex being 66 and Atripla 00 a month)