You can't connect it to two computers at the same time, but you can connect it to one computer to back it up, and then reconnect it to the other computer to back it up.
The closest you can come to connecting it to two computers at the same time is to either (a) attach it to one computer and share it to the other computer over the network, or (b) use it in a NAS (Network Attached Storage) appliance which is accessed by two computers (which is really just a fancy way to do (a)).