bradshsi:
You've obviously done a fair amount of comparison with your 2 computers. As you say having the same algorithm helps. Even so given that both the algorithm and how it is programed have an effect, you can't be certain that the performance of both computers will be consistant 100% of the time. At best you can only be "very sure".
Though personally if I was to use computer(s) (which I don't for previously stated reasons), I'd find your approach logical for risk mitigation.
I'd have to disagree with that statement. You rely on your own computer(s). You can't be sure your buddy is on the same profile as you are. If both your computers fail you call the dive.
Thanks for the informative post. I've enjoyed reading the different viewpoints on this thread.
One thing that makes a computer handy is for consistent performance of repetitive calculations in a relatively short period of time vs. use of pencil & paper, or even pocket calculators. I'd have to say I haven't noticed inconsistent behavior in the Dive Rite NiTek3 I've used for 5 years nor the TUSA IQ-700 I've used for a year.
Typically I've seen double redundant or triple redundant instrumentaion deemed sufficient for things like aircraft and petroleum process equipment - ISA standards (last I looked) pretty well supported this on the odds of simultaneous failure via statistical analysis. Automobiles rarely employ any redundant instrumentation and the single most critical component for either safe or less than safe operation of said vehicle is still the nut behind the wheel - no different than before computers were used in automobiles.
My insurance agent's firm I'm confident uses the same principles in math - my rates are lower not because they can definitively say they'll never have to pay a claim from me, but they can be reasonably sure. I'm pretty sure but never 100% that a damaging earthquake might not strike my residence - but I can be reasonably sure it can and will be affected by tropical weather systems from time to time but never 100% certain exactly when and exactly to what degree. If you get right down to it, according to the Heisenberg uncertainty principle, we can never be 100% sure of anything (I guess even including the validity of said principle - I'm not sure if that reinforces it or weakens it philosophically).
Since my wife is the photographer and part of my diving fun is helping find things to photograph (as well as see what she's found to photograph on her own), our dive profiles look really similar when the data is downloaded from our dive computers and plotted using tools like MS Excel on our home PC (which with Windows OS software I find far more inconsistent behavior on the PC machines I've used over the last 12 years than any dive computer I've used in the last 6 years). So my risk analysis assessment is I would probably be in worse shape if I didn't check with her instruments if in the unlikely event both my dive computers simultaneously failed. I don't mean we would continue the dive as if nothing had occurred if this was the case however; we would safely end the dive (which concievably could include a recommended deco stop and at least use of a functioning time device for the safety stop - although we could count off the seconds individually, but not be assured we were synchronized in counting them). We are buddies (in life) and she must like me for more than my life insurance policy value as we stick together, have made it back safely from every dive thus far, and she has half the gas consumption rate I do! And, we've never had a single dive computer fail on us during a dive - but we have a plan (and equipment) for contingency is the main point for us as you mention. Saw no reason to eBay the older model with the more limited dive memory (yet). If we move to rebreathers next the use of triple redundant O2 sensors in recent models would fall in line with what we're accustomed to for such critical instrumentation - to be honest, we were surprised such apparatus made such extensive use of mechanical parts rather than electronics until recently!
I suppose a big enough electromagnetic pulse (like from a thermonuclear detonation) could scramble all 4 dive computers simultaneously but with the shielding of water for EMP we'd probably be in boiling water at best if not vaporized if we were close enough to such a pulse during a dive, and we wouldn't be worrying about DCS at that point.
I appreciate your viewpoints as well - we can agreeably agree to disagree, based on our own analysis of how we choose to do things - something not always easily achieved! I'm not recommending anything for anyone, just following the title of the thread with providing info on what I do.
Happy & safe diving!
You're quite astute! 
