Alter-egoes is a technique where two or more players play basically the same character at once. There might be minor differences between interpretation of the character but its position in the game and its story is the same.
This technique is used when you want to mirror the story in the game multiple times at once. You can use it to emphasis some aspekts of the story. Each alter-ego has its own net of co-playing characters and relationships and by other players, they are seen as different characters, often best friends. Together towards others, they can share a conversation (and switch with each other) or they can shadow each other and influence the game by suggestions to each other. Dialog between Alter-egoes serves as internal monolog of character and can be used to sort out thoughts about developement of the story and figuring out next actions of characters.
This technique was used i.e. in Cien Años de Soledad larp to emphasis the repetitivnes of the story, which playes imporant role in the mood of the book the game was based on. There were three alter-egoes of each character in the game, so the story of the game was played three times at once and the game was focused on one family, present in te game three times.
Another example is Valley of Shadow, where there were two Alter-egoes of each character present and they were guiding each and sharing the same fate.