In order to be able to burn the same thing to multiple drives, they must all burn in the same strategy and the exact same speed. Only identical drives with identical firmware can do this. Burning from separate instances of Nero does not require this. This is also the reason why duplicator towers have all identical drives.
As for different channel do you mean separate IDE cable? I have two ide channels (usual) and of course you can connect two per device (master and slave). Does that mean you can only do two CD-ROMs to get the one per channel?
IDE limitations require that you can only access one drive on a channel at a time. Thus you cannot burn to one drive (say...the master) and simultaneously burn or read from the other (slave). Most IDE controllers are fast enough to allow burning the same thing to 2 drives on the same channel
(2 drives in the same instance of Nero). The controller is actually switching back and forth between the 2 drives, but it's fast enough to avoid buffer under-run.
So, if you have 2 IDE channels, (and no hard drives on them), you will be able to burn from 2 instances of Nero to one drive on each channel. Or, possibly from 2 instances of Nero you could burn to 2 drives on each channel (again presuming they are 2 identical drives).
All the above operations do also require that somewhere you have a HD that is capable of delivering a sufficient datarate for multiple burns. With DVD's this is usually only possible at lower burn rates.