It's maybe not practical, but it is not impossible. You can build a four-component system that does the job. You build two computers with different ISAs from vendors you assume to not share a backdoor, and let all I/O happen through a simple verifier, that copies the input for both machines, compares byte for byte their output. For cryptographic purposes, the random number generators of the machines must be linked, which would be our forth component. The software for each computer should be derived from different sources, which must follow the same I/O specification.