You can't use MSN messenger with two accounts at the same time. but you do able to change between the accounts every time you connect, just uncheck the option to remember your password.
Another thing you can do if you want to use two accounts at the same time, you can install and use another program that lets you connect to MSN service (like Miranda IM (my recommendation), Gaim, or Trillian) with one account and the official MSN messenger with the other one.
