I was able to get a friend to join, and I'm currently helping him learn the game.
He comes over a lot and I wanted to know if he could log into his profile on my computer, or vice versa.
If he can, do we need to sign up as multi's or will him logging on from my computer and vice versa every once in a while not cause any problems?
You need to sign up as multis any time you will be logging in from another person's connection. If you know that it's never going to happen again you can then unregister.