It doesn't look like there's an official rule for this, except for the rule where one Medium sized fighter fights from one 5' square.
So, I guess, if you've got 10' of space in a corridor that you can put two Humans, one in each square, each equipped with a Reach weapon with no problem.
I'm not sure if I like that.
I'm thinking of allowing it with Piercing reach weapons, but not with Slashing or Bludgeoning reach weapons.
Therefore, if a Human has a polearm that only allows for slashing damage, and his reach is out to 10', then his comrade can't stand in the adjoining square. That polearm will be swinging around, slicing through the air, as the attacker moves around in melee.
Thoughts?