Two Ipods and two accounts on 1 computer

Im on vacation and my sister got an ipod for Christmas. There is a family computer, which has my mothers ipod connected to and the itunes software. My sister got the new nano but is running into conflicts with my mothers ipod. I dont use ipod because I hate apple products so I know nothing about this stuff. So my question is how to I set up two ipods and two itunes accounts on 1 computer?