Yes you can if your computers motherboard has dual PCI-e x16 interface card slots. if you have an older motherboard with only an AGP graphics slot then you might still be able to obtain a PCI graphics card. If you motherboard only has 1 x PCI-e x16 slot then it might have a PCI-e x1 interface slot and there are a couple of x1 PCI-e graphics cards available.
what motherboard / pc do you have? I,m presuming you are asking this in responce to your other post about external graphics cards for laptops.