[Hope Springs Hauler]

Actually, the mechanics of materials applies no matter what it is. The moment of inertia and section modulus are dependent on the distribution of the material from the centroid.

The exact example that was originally asked about was using 2 pieces of steel and sliding 1 into the other and passing it as equivalent strength as a single piece of steel that has the same OD and wall thickness.

With my example, you brought another dynamic into it. When you screw/nail the 2x4's together, you are beginning to create a composite section; much like glu-lam lumber. Screws or nails that connect the sections experience a shear force that is generated along the interface and thereby connect the two pieces into a single composite section. For that situation to apply to the sleeved tubing, it would have to be plug welded in many different spots along the circumference and length so that the 2 pieces act as 1.