I agree; I'd be quite surprised if it were at all common for separate charities working in the same field to be so well-balanced in scale that proportional contributions to both outweigh contributions to just one. Since there's no clear feedback mechanism to help people maximize expected utility in giving, there's no reason to expect the MUs to be anywhere close. Therefore we should strongly expect that a "bullet" strategy will outperform diversification. I dub this the Inefficient Charity Markets Hypothesis.

On the other hand, I wonder what percentage of charity contributions are given by the top 1% of donors, people who really can make a dent in these problems? Their impact probably dwarfs anything the vast majority of small donors in the audience would do. But I'd bet they're smart enough to realize this stuff doesn't apply to them.