Actually, both of them are the same person. Sakine Meiko is illustrated as young MEIKO who made music debut at 15. Unlike MEIKO, Sakine Meiko is a little naive and is not addicted to alcohol. ^^;
While MEIKO is an official Vocaloid 2 character, Sakine Meiko is actually made by fans, and officially recognized by Crypton Future Media, the publisher of Character Vocal Series for Vocaloid 2.