[mr_person]

Tandem (now HPE) NonStop do something like this... Run 2 systems in lock-step for super resilient computing. The code has specific instructions for validating operations by comparing the outputs on both CPUs

[082349872349872]

Originally Tandem (despite their swag coffee mugs that had only two handles) ran 3 systems; having an odd number made it easier to compare outputs.

https://en.wikipedia.org/wiki/File:Tandem_Mug_with_Redundant...

[denton-scratch]

Also Stratus: Wikipedia says they were originally built around M68Ks. The WP article is a bit thin on Stratus's early products, but my vague recollection is that they used 3 processors in lockstep, with voting to detect a faulty processor.