1) You cannot have a hard drive which is simultaneously attached by USB or FW to two CPUs.
The way to do that is to make the drive a network-shareable volume. You do this either by
a) attaching it to a computer (the stationary PC would be the obvious choice), and using File Sharing to share that volume out to the other computer over the network
or b) buying a NAS (Network attached storage) hard drive that has its own tiny server and ethernet connection built in, and acts as a stand-alone server.
2) iTunes is run on the computer, not 'on' the data drive, so installing iTunes on a shared drive is not helpful. It may be you meant, install the iTunes libraries on a shared drive.