My Bobby is FIV +. As long as they don't fight, all will be fine. FIV is not easily transferred. Mine share water bowls, food bowls and litter boxes no problem. You don't want them fighting; if this means taking an extra few days to get through the transition phase, do it as long term you will be glad you did.

My Bobby is such a sweetheart, a real cuddle bug not only with me, but also with the other cats and with the dogs as well.