I have an object that is composed from two child boxes.
That object have some code that is run on OnMouseUp on the colliders.
I can add one box collider to the object but unity won’t let me add another.
If I add each collider to its box OnMouseUp on the parent won’t be called.
How can I have a collider composed of two boxes on the parent?
Look at the part about compound colliders: http://unity3d.com/support/documentation/Components/class-Rigidbody.html
Briefly, put OnMouseUp in the main object script. Events on the kids run on parents. Use or don't use the one Collider on the main object (for example, if you can't line it up, the main can have no collider.) Kids are empties, no scripts, with just colliders.