How to install Hard Drive and CDROM with the same IDE cable?

The links rj713 provided you looks like it has some good information. Part of what you want to do is hardware and brand related. I suggest you read the installation/owners manual for a drive before purchasing it. They are usually available on the manufacture's web page for the product. Most CD/DVD drives want to be on the primary/master position of the IDE cable and have the jumper set to cable select, sometimes master. Some hard drives/optical drives don't work well set in cable select if there is another drive on the same cable others won't work unless they are in cable select. To a large degree it is a matter of setting the jumpers in a way where the drives don't fight. Your primary drive should always be set master or cable select. Your secondary drive should always be set slave or cable select. There is no guarantees that your drives will cooperate with each other, for various reasons, this is known as hardware conflict. More times than not, you can play around with the jumpers and position on the cable to get them to like each other.