I was diagnosed with Syphilis a few years ago. Recently I found out I had HIV. When the doctor ran through my tests the Syphilis test was negative. I was told by someone that a test can not be negative if you have tested positive. Is it possible that I never had Syphilis even though they treated me for that? My partner tested negative and the two of us have been together for about four years.

It depends on which blood test for syphilis was done. If an FTA was done, it will always be positive if you ever had syphilis at any time, whether you were treated for it or not. If an RPR was done, it will be positive if you have syphilis that has not yet been treated. So it is possible to have had syphilis, received appropriate therapy for it, and now have a negative blood test if that test was an RPR (which is the test most commonly used to screen for syphilis, by the way).