Also, more practically, two computers with different ISA and underlying hardware that compute the exact same high level semantics, that don't know each other but transparently share the necessary hardware (for example hardware random number generator), talking to the world through a simple electronic checker, that stops the system if both computers don't communicate exactly the same information bit by bit, is also pretty safe, even if you use backdoored computers. Just make sure both computers don't contain identical backdoors (which is not that difficult).
High and sufficient security in computer systems is practically possible. We just don't work at it. Instead we work on JavaScript and WebAssembly and proprietary hardware and software.