I am not sure if you could use the USB to one pc and the ethernet to the other so you could use either independantly or not, but it could work maybe.
There wouldnt be any harm in trying it
The normal way would be to connect via the USB connection to your main pc, unless it has 2 ethernet ports then use the ethernet connection, then use a crossed ethernet cable to connect both pcs and set up a network using the main pc as the gateway.
You could then have both pcs on the net at the same time and transfer files and share printers etc.
You would have to have the main pc switched on to be able to use your second one but could connect to the net from the second one without having to connect on the main one first, as long as the main one was already switched on