you can copy from one cd to another but only need one burner for that a regular cd/dvd rom would work as well. But just about as quick to copy cds without the second drive (nero disc copy ...etc) which copies if to hd then to disk.
Only reason to have 2 burners is so you can burn 2 disks at the same time - not something I do regularly...
A direct CD/DVD-to-CD/DVD copy can be done with 1 burner and 1 reader, but most if not all burner software can do a copy with just a burner - it reads the source to the hard drive then you swap in a blank and it burns it.
So, really, only 1 drive is needed - a burner, since it can read too.