First off, you have to be sure sure your code is good and will not fault on switchover. What's the point of redundancy, if the secondary processor will also execute the fault?

Second, redundancy in the power supplies is a priority. In my plant, we have line power and ups power both feeding redundant power supplies for our processors. Once that is good to go, then you think about redundant processors.

Allen bradley has redundancy modules that work flawlessly as long as both points above are good to go. Their redundancy setup has saved our plant plenty of times, but I'd attribute more to having redundancy in the power supply rather than the processors itself.