I can edit colliders by holding down the shift key. But when i add a second collider, the shift key always selects the first one. And there is no "Edit Collider" button in the editor (Win/4.5.4f1). I'm testing it with two EdgeColliders2D.
In Unity 4.6 there's a dedicated button in Inspector labeled "Edit Collider" that you could use to edit the collider.
One GameObject can only have one BoxCollider, just choose GameObject you want edit and modify your box collider