[ashtonbaker]

I think you're saying that PADI's recommendation that "you should have a backup plan" means implicitly that you should dive with two computers. They wouldn't make that an implicit recommendation, they would make it an explicit one. Which they do not. At least not for open water recreational diving.

The backup plan for recreational open water diving would typically be to end the dive if your computer fails, not to continue computerless. At the point that your computer failed, you should have been within no-deco limits, which you can confirm with your partner's computer to a reasonable margin of safety, so you can ascend safely.