When you are online with Yahoo Messenger and at the same time sign-in with your yahoo mail account, Your connection with Yahoo Messenger will be lost(and it will sign out automatically) when you tried to open your mail chat.
Or if your status is " Available " on the mail chat of your Yahoo Mail account. Once you sign in on this account, you will be signed out in your Yahoo Mesenger account unexpectedly.
So, here is what you need to do.
Sign in with your Yahoo Mail account. On the upper part of the page, beside your name, click sign out of chat. The message should appear "offline"
Now try signing in with your Yahoo Messenger account. Then after signing in, view the content of your yahoo account. It will not disconnect the connection of your YM.
Let me know if that solve your problem
. Please come back and give me some ratings.
Thank You for using Fixya!