To use both at the same time you will have to connect both to the motherboard at the same time. Find out if your drives are IDE or SATA.
IDE ports can accept two connections (one master and one slave) on the same port, you will be able to plug both drives in using an IDE cable and both will work at the same time.
SATA is a single channel and if both your drives are SATA you will need two ports free on your motherboard, once they are both plugged in they with work at the same time.
you can have one drive on IDE and one drive on SATA and they will both still work at the same time; you will also have to ensure they both have power from your PSU or the motherboard will not recognise the devices. When both devices have a connection to the board windows will detect both drives which can be viewed in "my computer".
Teris, April 2010