How it will work best for you will depend on how you use the drives most commonly and where you want the best performance.

The deciding factor for performance (ie transfer rate) is the fact that each IDE channel can only access one of the devices on it at a time. So you should try to have the two devices you copy between most, on different channels.

Usually the best is pretty much as you have it, DVD-ROM as Primary Slave, CD burner on secondary, preferably as Master, although it may not matter in practice. That way the source CD can be read while the data is being transferred to the target. When you copy CDs on-the-fly, you should get the fastest result, and it will also give the fastest copying from CD to the hard drive on the other channel, and between hard drives.

Also this configuration avoids the occasional conflict that can occur between two optical drives on the same IDE channel. This seems to happen more often than conflict between hard drives, or between a hard drive and an optical.