First, install Yahoo messenger 11 and then download this: Yahoo Messenger - Multi Messenger Registry Patch (0,46 Kb)
it contains 2 *.reg files, Install.reg and Unistall.reg
To make your Yahoo! Messenger 10 (or 11) become Yahoo! Multi Messenger, simply double click Install.reg
I just installed Windows 7 and Yahoo Messenger 10 and wanted to use multiple instances like I did on XP with YM 9. Thanks!
If you're unsure if this is a virus and will corrupt your system or not, check it with your antivirus first (it's clean). If you're still unsure, right click on each file after you unzip them and choose 'edit' to see the contents inside. You can see it just changes one value for Yahoo Messenger in the registry so it's perfectly safe! The 'uninstall.reg' just changes the value back to the default. Thanks again! Just leaving a comment for others so they know this works fine.
Tested on Yahoo Messenger 18.104.22.168-us (April 12 2013), 5 Yahoo accounts at the same time...
click on image to enlarge