Opening multiple instances of Messenger is known as 'polygamy'. Several patches exist to make this possible. The
A-Patch, the
Mess Patch and
StuffPlug NG are the most popular tools for this. Look at them and decide which one has your preference: the first two are patches you apply to Messenger by running them once and selecting the desired options. StuffPlug NG is a plug-in for Messenger Plus!.
To enable polygamy with the A-Patch and the Mess Patch:
download, run them and follow the instructions on the screen to enable polygamy.
To enable polygamy with StuffPlug NG:
In the contact list, go to Plus! > Preferences > Plug-ins and select StuffPlug NG. Click the Configure button and enable polygamy.