ideally to me means best performance. Having both on the same cable causes anything reading from the optical and writing to the hdd to wait for the bus to clear. ie - read cd, write hdd, read cd, write hdd... effectively cutting your performance in half (of the slowest device on the one cable*)
whereas having them on separate cables allows both operations to occur simultaneously.
* having two devices on one ide cable forces the controller to the interface speed of the slowest device on the cable to maintain compatibility between both devices. foe example, stick a PIO only optical drive on with a ultra dma drive and both will drop to pio mode because the optical can't "speak" dma.
another issue with having both on the same cable is even if both drives are dma if the optical experiences some read errors the interface will drop to pio mode, thereby slowing your harddrive down at the same time. It does not automatically raise it back up to dma mode. Have the optical drive on the second channel, the drop to pio wouldn't effect the HDD. I'm sure this is ringing a bell with alot of our older users.