Missing DVD drive

I have 2 optical drives installed on my computer, one Plextor DVD burner and one Memorex CD drive. For the 2 and a half years that I have had this computer I have not been able to get both drives to show up at once. It seems to choose the visible drive at random each time I try a new setup. I know both are receiving sufficient power and I believe are connected correctly. The problem has persisted through many re installations of Windows XP and I am certain that all of my other hardware is functioning properly. Have I missed something in the BIOS? What can I try next? Thanks