only thing that comes to mind is using different connectors, so have a drive with usb and e-sata or firewire and then plug usb on one machine and then the other on the other machine, no idea if it would work, kinda doubt it.
or just get another cable and switch it over as necessary without having to go down the back of the machine, not exactly a solution.
last thought - get a proper nas device which cost a little bit more than a external hd and plug that into your router.
^ Wouldn't work, imagine two computers updating the MFT at the same time or trying to use the same sector for different files...
Also using a USB switch (they do exist) is asking for problems, you would have to GUATANTEE that a safe remove was performed before switching.