it can be done but not recommended. supposedly runningb an optical drive and a hdd on the same channel produces interference. im not too sure about the accuracy of this but in an old computer i did notice i had some problems when i had a cd-rom and hdd both on the primary (i only did it because at the time i didn't have enough cables). also it is a good idea (like the 1st post) to get rounded cables they not only make things a lot easier but they make for more efficient cooling
If you put them both on the same cable, and say you wanted to copy something from the DVD burner to the HDD, than they cannot work on the same time. The DVD burner will read it, than it will be written onto the HDD. They cannot read and write at the same time. If you have them on seperate cables, than they can read and write at the same time.