Run regedit as Administrator and navigate to the following key HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows Live In the left pane, right click Messenger and create a new DWORD called MultipleInstances.
Next, right click the newly added registry key and choose Modify
Set its Value data to 1. Close Regedit.
You will be able to allow polygamy or run multiple instances of Messenger at the same time signed into different id’s, on one computer.