It depends on how you run them. I have two drives in my computer, a Seagate 250GB and a WD 320GB. By having two of them in my computer it isn't any faster (or slower for that matter) then if I had one. You are probably asking about setting up an AID0 (AKA RAID0) array. This requires either a motherboard with a RAID controller that allows this, or an add in PCI/PCIe card. I usually tell people to avoid AID0, as its not worth the headaches, risks, nor can most of us use the speed.