So you want it plugged into one computer but accessible by both?
The only way is by a network share.
You could have a crossover between the two machines that bypasses the LAN. You would need 2 network cards per machine. That's a pain and would require changing the metric on the 2 NICs on each machine. I can do that no sweat, but explaining it to the uninitiated causes some sweat

. The benefit would be dedicated 100/1000mbit between the two, once configured.
You can't have one USB device somehow plugged into 2 machines, or you will run into file overwrite issues and possible catastrophic data corruption, among other things. You can however do something like a USB switch, which behaves like a KVM.
USB 2 port sharing switch for sharing usb devices between computers and laptops
Still, it is one computer at a time, but you don't have to finagle around with plugs. Just push a button and computer A has it. Hit it again and computer B has it.