Yahoo Messenger (YM) is one of the most used to chat. Beside YM, people maybe used MSN, ICQ, Skype etc. Here i will give some tutorial how to login more than one account YM in 1 computer (just for Yahoo! Messenger).
Let’s Begin

1. Run registry editor. Start Menu -> Run, then type regedit and press enter.
2. Find this folder in the left of registry menu HKEY_CURRENT_USERSoftwareYahooPagerTest
3. In the right panel, right click mouse and choose New - Dword Value
4. Type Plural
5. Double click that Plural and filled it with 1

It’s finished all :D. You can sign in with 2 Yahoo account in 1 computer.