I think the Enterprise version supports several burners, and the retail version 2 burners.
There are also plugings in the web for up to 7 and 32 burners.
If you want multiple burning only for audio and you don't mind the price, I recommend to see Feurio Professional. It was designed for multiple burning from the beginning, and it stresses the computer less than Nero, mostly if you use Win98/ME. Although the most important point is the IDE / drives performance.