You can only use one Yahoo! ID at a time on your Yahoo!  Messenger, but you can create different aliases (also called identities) for yourself as a part of your main Yahoo! ID. Each alias/identity has its own profile. A profile includes additional information about yourself that you would like other people to see.      

      For example, you might have a Yahoo! ID of "janedoe." As a part of this main ID, you might also create the alias  "blueslover" and  another one called "janiac." You could use "blueslover" to talk with  your friends, and "janiac" to talk with  your co-workers. You can  also set Messenger so "blueslover" appears online and "janiac"  appears offline, or vice versa, or so that both  appear online at the  same time. 

To view a person's profile, highlight the name of that person. Then, right-click on that friend's ID and select View Profile.

To create a new alias for yourself, click the Login menu in the Messenger window and choose My Profiles. Then, click  the Create/Edit My Profiles button. When creating an alias, please keep the following in mind:

• You can only choose a name that has not already been taken.

• You can create up to five aliases as part of a single Yahoo! ID.

• Once you have created these aliases, you can choose which one to use when adding a friend or sending an instant message.
  

Turning aliases "off" and "on":

To control which of your profiles appears as online or offline at any time, click the Login menu and choose My Profiles. Removing the X from the box next to any profile will cause that alias to appear as offline.

To control which alias you use to add a friend, click the Add button. You'll see your Yahoo! ID listed next to step 3. Click the down arrow to the right of your ID to select an alias from the list.

To control which alias you use to send a message, double-click a friend's name to launch a message window. You'll see your ID displayed in a box in the upper-right corner. Click the down arrow to the right of this and select an alias from the list.