Shoutbox

There's ways you can kind of check if two people are on the same computer.
Do the following;
Start > Run > cmd > OK > netstat -n > return
Make a list of all addresses there, then send a file to two suspects, make sure they have both accepted and that the file is sending to both when you do this
Start > Run > cmd > OK > netstat -n > return
Two new addresses should pop up this time, if they are the same, it could indicate they are using the same computer, chances are if they are different they aren't using the same computer...but things that can go wrong with this (and it's very likely it will) are things like new addresses popping up when your computer opens a connection with another computer (like a download on a P2P client), you making a mistake in looking at the connections, files being sent over the server or different connection settings, or even the obvious fact that two people can sometimes have the same IP address...
This should never be concrete evidence...I understand what I said may not be totally clear, if you need help with any bit (or all ) just ask.

Edit: And the obvious solution of just asking them, and assume they are telling the truth