Re:
Hello,
You can have two Hard Drives in a PC simply by hooking one up to the Hard Drive Master IDE connector, and the other to the Slave IDE connector. Also, you'll need to either set both Hard Drives to Cable Select (the position of the drive on the IDE cable determines whether it is slave or master)...or set them individually to master or slave depending on where it is connected on the Hard Drive IDE Cable.
Next, it's simply checking BIOS to make sure that it has detected both drives. Then, if they are both new Hard Drive, you'll need to install an Operating System to one of the Hard Drives, on a new partition.
If you are simply adding a Hard Drive to your PC, and you already have a current one with an Operating System installed, then you'll simply need to hook it up to the spare Hard Drive IDE connector (usually slave), making sure that you've set it to Slave or cable select. Make sure that you have enough power from your Power Supply Unit, some spare molex connectors (which I'm sure you do)...and that the Hard Drive IDE cable splits into two connectors so it can accomodate two Hard Drives.