Yes, but
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.