Sharing a 5 by 5 foot square with another is not the same as standing in a 2,5 foot corridor: The corridor confines you to 2,5 feet with all the way, but if you and a friend walk close to one another in a 5 foot corridor, the confinement's only about 2 feet long. behind your friend, and in front of him, you have 5 foot of width again. Also, the two don't have to walk side by side, but one could be a foot or two further along the corridor, and still the two are in the same square. And I think they could well manage to make their own attempt to evade any harm without getting in each other's way to much. They'll maybe get a penalty, but it isn't impossible to move.