At first, you must make your design reliable. Since it make no sense to use a couple of CPUs, if all these unreliable.
Also for such redundant systems an odd number was needed (3, 5, ...) to make a majority decision.
Typically also different CPUs and different languages used.

But now some hints to get reliability at first:

- deliver a clean power supply voltage

- prefer internal code memory, since many times better proofed against any kind of EMI (for extreme reliability factory programmed ROM used instead Flash)

- deliver an excellent power on reset
(mostly the MAXIM parts known as reliable)

- use an always enabled watchdog (external, since most internal are sleeping after power on and possible do be disabled).

- protect, debounce all inputs and outputs (supressor diodes, debouncing software)

- also the pcb design and the housing are importand.


I think, there are many other points.
And forget not to make the software reliable. E.g. on state machines decode every state, also such which looking impossible to be reached.


Peter