Have done this myself.
It is not recommended to have two CD dives on the same cable.
Therefore, connect one CD drive to IDE 1 using the same cable as the hard disk drive (make sure it is set to slave) and the other CD drive set as a master and directly connected to IDE 2.
Which CD drive you connect to which doesn't really matter ad they will perform the same either way round.
If your existing CD drive is on the same cable as the hard disk, just set youe CD/RW to master and plug it into IDE 2,