I'm afraid what your trying to do won't work. firstly you can't mix drive manufacturers (or even models in some cases), because they will use a different writing strategy from the other drive, and therefore read the data at different times from one another. Secondly, from what many here have learned, the software (all of them) haven't been optomized for mass duplication, its a joke really that it hasnt' been done yet (afaik anyway). Also your using an old PC, I'm guessing you PSU is not up to snuff either, when a cd/dvd drive spins up it uses a good amount of power, not sure 10-20W each.
Your best bet is to make duplicator using a dedicated controller (without PC), they sell them on ebay, and copystars.com, get a decent PSU as well.