GrapeVine is a mod designed to make it easier for multiboxers to communicate with people that don't know which character to whisper. Simply install Grapevine on the clients of each multiboxed character (including the relay target) and add your relay targets to the WoW Friends list.
To add a friend to the Grapevine list, type /gv add [name]
Then when anyone whispers one of your characters, the main character will see the relayed whisper.
Slash Commands:
/gv add [name] - ticks [name] on current character's friends list if it is there
/gv remove [name] - unticks [name] on current character's friends list if it is there
/gv removeall - unticks all names on current character's friend's list
/gv list - views friends currently in Grapevine's relay list
