Its not just the 2 more bars, the cage is diff. design and has smaller tubing. My buddy has a 2 bar and it is way different than my 4 bar..
The main thing, and you can see in the pics you can see the weld fail, is at the top of the b post and the roof bars. The 2 bar is just gusseted and the 4 bar has a tube / brace triangled of the b post and the roof bar.