I wrote a little wrong. If I understand correctly (your picture becomes quite small on my computer...), your floor joists rest on your "homemade beams." What I meant is that the load from these exerts a greater pressure on the inner beam. As you say, the inner beam will give somewhat, and force will be transferred to the outer one, however, you must have extremely stiff floor joists (and preferably not too stiff tie rods) for them to be able to take an equal part of the load. It certainly helps somewhat, but I don't think you can fully account for the outer beam without screwing them together.
