Tell me if I understand this argument correctly: under reasonable values for marginal dollar effectiveness and utility of each charity, your donation probably won't push one of the charities to the point where the marginal utility of donating to it drops below that of donating to the other. That is, your donation, even if spent all at one, will Probably not pass the point of relative diminishing returns.

If I have it right, please add the nonmathematical summary.