Many tandems have a front bottom bracket that looks like this:
The front bottom bracket is mounted off-centre in an aluminum shell called "the eccentric." When the pinch bolts shown in the picture are loosened, the eccentric can rotate within the bottom bracket shell to tighten or loosen the tandem's timing chain (that joins the front and rear cranks).
If the pinch bolts broke, or if the braze-ons they fit into sheered off, would the bottom bracket shell pop open? It seems to me that the integrity of the bottom bracket shell is totally dependent on the bolts; that you'd bend the frame quite badly if they broke.