How do you connect two hard drives to one

You can easily connect two hard drives to one computer.You can add another hard drive to another port in IDE cable where the first hard drive plugged. thats it .one is primary and other is secondary.

If these are SATA drives then it's simple. You need an additional sata cable (maybe you got it with your motherboard). You just mount the drive inside your case with screws and then plug the cable into the sata socket on the MB (it is designed in such way that it can't be plugged incorrectly). Each socket is for one drive so make sure your motherboard has enough of them.

If you have an older power supply then it probably won't have SATA power connectors but standard molexes. Then you need an adaptor MOLEX -> SATA POWER CONNECTOR.

If the drive has PATA connectors (is Ultra ATA):
The drives are connected to MB with ATA FLAT (tape) CABLES. Each cable has three connectors: One goes to the MB and the other two are for the drives. If you already have an ATA drive connected then you can use the spare connector on the cable.

If you put 2 drives on the same cable make sure that you set the old drive to MASTER and the new to SLAVE (you do it by moving jumpers on the drive, it should be documented on the drive's label).
More info on UATA

After installing the drive if the os doesn't boot, make sure that MASTER/SLAVE settings are correct (if drives are UATA) and that your old drive has the highest boot priority setting in bios.

After installing the drive (it usually isn't done in the factory) you have to partition it and then format the partirions. A nice tutorial here.