1. Install MSN Messenger 7.5 (If you already have it installed you don’t need to)
2. Rename the MSN Messenger folder to MSN Messenger 2 (usually located in C:\Program Files\)
3. Install Windows Live Messenger.
4. Apply a polygamy patch A-Patch to both Messengers, or to the one you plan to open second.
5. Then change all the paths of the shortcuts to MSNM 7.5 to C:\Program Files\MSN Messenger 2\msnmsgr.exe.
Search next time, this has been asked many times.
Edit:
http://shoutbox.menthix.net/showthread.php?tid=54000