Aslong as Windows Live Messenger itself does not allow use of two different nicks at the same time with the same account this won't be possible.
What is possible is to get a different
Live ID (formerly .NET passport) and add one group of persons to one and the others to the second and use different nicks that way. Now with the built in polygamy of Plus!Live (Plus > Preferences > Messenger > Main) you can already log into several accounts simultaneously.