With the official Yahoo Messenger, it is not possible to login with multiple Yahoo IDs to Yahoo Messenger. If you want to login with multiple Yahoo IDs at the same time, then you can use Multi Yahoo Messenger. Multi Yahoo Messenger is a simple application that allows you to run multiple yahoo messenger instances simultaneously.
All you need to do is to download the application, install it and run it. First time you need to patch, when you open Multi Yahoo Messenger it shows Patch button and Restore button. Click on Patch to patch Yahoo messenger.

Now open Yahoo Messenger and login with a Yahoo ID, repeat this process depending on how many instances you want to run.

That’s all, enjoy the multi yahoo messenger logged in with multiple Yahoo IDs.

If you want to remove the patch, open Multi Yahoo messenger application and click the Restore button to revert the changes. Now the patch is removed and you can’t use multiple yahoo messengers at once.

Download Multi Yahoo Messenger