One difference between pregnancies and debugging is that the success probabilities of pregnancies are more or less independent -- couple A's failure to conceive

One difference between pregnancies and debugging is that the success probabilities of pregnancies are more or less independent -- couple A's failure to conceive doesn't cause us to believe that couple B will be any more or less likely to succeed -- whereas bug-finding times are not independent, because all of your developers are searching for the same bug. If developer A fails to find the bug in a week, then, assuming she is competent, the bug is likely very subtle and so developer B will likely also fail to find it. This doesn't invalidate the argument entirely; adding developer B is clearly no worse than just using developer A (assuming B doesn't disrupt A's work and also that we don't care about the cost of an extra developer), and, to whatever degree B and A are using different strategies (giving them some partial independence), it will somewhat reduce the tail probability that the bug takes a long time to find. But the lack of independence, relative to the pregnancy example, means that the gains probably won't be as dramatic as this post assumes.