If you have 2 av's running at the same time then it will cause you problems, I've seen instances where the 2 AV's reckon the other ones a virus, iv seen instances where one of them just rolls over and lets the other one do its job, shutting itself down, i've seen instances where they BOTH do this, leaving you with no AV at all.
Bottom line? Make a decision about which one to use, but dont use both