Can a tandem's front bottom bracket fail catastrophically?
First off, in order for there to be any sort of a "crisis", both bolts would have to fail, an unlikely possibility on a reasonably well-maintained bike.

But, beyond that, the integrity of the frame doesn't depend on the BB shell resisting twisting loads.  Rather, due to the "truss" nature of the standard bike frame (tandem or not), the load on the shell is almost totally "radial" -- force into or away from the shell.  While this force would tend to "pry" the shell open, it's very unlikely to be substantial enough to cause catastrophic failure.