Sounds cool.
The only drawback I would see is if the alter ego's started talking to each other and answering back.
Time to be scared----very scared.
So seriously now. Your thinking I assume to adding a few tables to w3t_Users to track and mark them main or alter and which user. Kind of like Parent Child kinda thing to track things and allow a toggle to switch automatically between the main and all alter's assigned to the UserID.
If I understand where you are going with this it would be great. I think it would cut down on trolls alot. Sometimes people want to express an opinion or such and do not want to express it themselves. Maybe only ADMINS and MODS know which alterego's belong to who.