To enable polygamy in Messenger Plus! Live, go to the Plus menu > Preferences > Customize! > General, and check the 'Allow me to connect to several accounts simultaneously' option under the Multiple Accounts section. Click OK then start Messenger again by double-clicking its shortcut on the desktop or start menu. You can open Messenger as many times as you like, and sign in with a different account for each instance.
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CookieRevised's reply to How to use/close MSN Multi-messange functions?.
