You can do that no problem, but the comb will only run at the lowest speed of the two, ie. HD is UDMA mode 5, CD-Rom is UDMA mode 3 then the combo will only run at the lower setting of Mode 3. To get the best performance put them on separte IDE cables. Also make sure you mobo supports ATA100 (UDMA Mode5) if it doesn't then it will only run at the max setting supported by your mobo.
Ep, glad to see you come back and tidy up...did want to ask a one day favor, I want to enhance my resume , was hoping you could make me administrator for a day, if so, take me right off since I won't be here to do anything, and don't know the slightest about the board, but it would be nice putting "served administrator osnn", if can do, THANKS