> The self-checking pairs ensure that if a CPU performs an erroneous calculation due to a radiation event, the error is detected immediately and the system responds.

How does a pair determine which of the pair did the calculation correctly?

It doesn't have to. It raises an error that the system can detect and take action on. Usually that'll be some combination of interrupt/reset and an external pin to let the rest of the system know what's happened.

In simple terms, this works by doing an XOR on the outputs and if they disagree, performing a fault recovery.

There's also space systems that use 3 processors and a majority vote for the correct output, but that's different.

You just run the calculation again until both agree.