It happened in the past that I wanted to run multiple Yahoo ID from the Yahoo client. (might not be working anymore in yahoo version 9 or/and 10)
Open the registry: start / run / type regedit and enter.
Navigate through the registry to HKEY_CURRENT_USER\Software\Yahoo\Pager\Test
Create a new DWORD key called ‘Plural’
Add as value ‘1’
Close the registry and now you can start several yahoo windows using a different’s ID.