Actually, with all CD-RW's using burnproof (or similar) nowadays, it is not necessary to have the burner on a separate channel from the reader. And as HDD speed is more important that optical speed, and as opticals can slow down an HDD, you should put the HDD on it own channel.
HDD = primary master, CD-RW = secondary master, CD-ROM = secondary slave
Now way back before CD-RW's had burnproof (and a slow read meant a coaster), I would have said otherwise. But nowadays that doesn't matter and speed emphasis should be given to the device used the most, the HDD.