Thats easy as pie, install Windows Live Messenger and sign in, then open Windows Messenger which works with me and sign on, and enjoy using two id's!!! Or if you have MSN 6.2 or something like that install it and apply a patch downloadable from http://www.funmaza.com/ and use it for signing on with unlimited accounts at the same time!!
It is possiable if u use any other messenger client other then msn messenger.there are tools like trillian messenger, by using this messenger client u can use muliple accounts.