On putting all one's charitable eggs in one basket:

I note that GiveWell recommend splitting one's charitable giving between their top charities in a certain ratio. But it seems that this would reduce the expected value of one's giving. Is this considered by others to be the best way to donate, or is it better to give all of one's donation to that single charity estimated to be most effective? I imagine this is the sort of thing that has already been discussed, so pointers to any previous discussion would be of use.