You're looking at a RAID or JBOD Array. First of all, what type of computer are you using? If you have a decent motherboard, you can probably acess your BIOS and set this up, but you'd lose all the information on them once you set up the array. You'd have to back everything up somehow, first...
RAID 0 (Striping) combines the drives and splits your data between the two disks so that it has better read/write performance, but if one of the drives fail, you'd lose anything that was stored on the array. RAID 1 (Mirroring) combines them and then writes a copy of the same information to both drives, so that if one drive fails, the other can be used to retrieve your information, at a slight performance cost JBOD, or "Just a bunch of disks", combines them into one disk, without doing anything extra - it just adds them together.
In RAID 0, your capacity is the smallest disk size times two. In RAID 1, it's just the smallest disk size. And in JBOD, it's both disks added together.
More information on this is in the RAID section of the storage forums.
Good luck! I'm sure more people can help you out if you need it.