Gigabit ethernet is faster than firewire, (1000 megabits vs. MAX 800megabits), but you have to keep in mind that no single HDD actually gets close to even 800 megabits, so you wont loose any performance by going for either. I would go for the one thats the most convenient and the cheapest.
As for the original question, dont think you whould be able to use 2 interfaces at the same time, might be able to use firewire on both computers though, not 100% sure about that one.
edit: seems there might be a slight miscommunication, Im saying that a HDD can transfer more data than the FW400 bus can handel, but also that the FW800 and gigabit ethernet busses are faster than a HDD, so FW800 or ethernet, either way you will get max HDD performance.