Kindly's comment gets it right. It's not about satiety. If you're a consequentialist and care about the total amounts of money donated to each charity, rather than about how much you donated, then the decision in favor of the equal split must be very sensitive to the donations of others like you. That's the relevant difference between selfish spending and charity.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.