Why don't my edgecollider2d's react to each other?

I have a few sprites with edgecollider2d colliders with each a rigidbody2d component. I would expect them to fall on top of each other, but the edgecollider2ds just fall over each other - i.e. they ignore each others existence. Other colliders do react to the edgecollider2d’s. What to do?

Found the answer in the Box2D manual: A major limitation of edge shapes is that they can collide with circles and polygons but not with themselves. The collision algorithms used by Box2D require that at least one of two colliding shapes have volume. Edge shapes have no volume, so edge-edge collision is not possible.