If this is your POV, then you probably should not keep any fish at all. Adding any fish brings a certain amount of risk, no matter the size of the tank.
And the main point here is understanding the risk.
Never keep any fish at all? Really? Read more carefully. I think you completely missed the point of my post. I'm not saying that it can never be done, and never should be done. I acknowledge the fact that many have done it, but also know that many have had failures trying this also. While it is admirable that the OP is asking if it cane be done, my opinion is that if a person is asking, then they do not have enough information to attempt it. Someone who has done significant amounts of research on the issue would know the pros and cons, and know the ramifications of their actions - whatever they may be. I personally do not agree with mixing two species of clowns in one tank - but I think I know enough about it to be a "devil's advocate" and argue that it could be done (and what circumstances that would be). While there is an inherrent risk anytime a fish is added, why add fish that the risk is very high to begin with - especially for someone without the prior knowledge base to know what they are getting into. If you would prefer, my original "rule of thumb" could be appended to add: "In the mean time, read, read, read, and when you can argue both sides of the issue, then you are read."
Again, I doubt my success with two pairs of Clowns is unique. People generally only post when they have a horrible experience. Rarely do people post successes.
To that point, how many more don't post their failures due to a) not even being on RC (remember, we are a small fraction of those in the SW hobby), b) embarassment, or c) not even really carring? This last point of yours holds little weight IMO. I think with such a topic that people seem eager to chime in when things against the norm actually work, I would expect more people to say it worked for them - if it actually did. What you have is the exception to clownfish behaviors in aquaria, and shouldn't be viewed as common, or even acceptable to attempt until all known factors can be accounted for, risks mitigated, and exit strategies planned for.