Um... that's not how I read it. It's about controlling the failure modes. The pane with the hole is the "backup"--without the hole, it would be stressed rough

Um... that's not how I read it. It's about controlling the failure modes. The pane with the hole is the "backup"--without the hole, it would be stressed roughly the same as the outer pane during normal operation. This has a major consequence: when one breaks, the backup will probably be in the same sorry state as the primary before it broke, so it will break soon too! Not a very good backup! Sure, because they are sharing the normal load, it is less likely to break in the first place. But, you'd have essentially doubled the strength, but removed the backup. That's not a good trade-off.