The logic that you should donate only to a single top charity is very strong. But when faced with two ways of making the world better there's this urge deny the choice and do both. Is this urge irrational or is there something there?
At the low end splitting up your giving can definitely be a problem. If you give $5 here and $10 there it's depressing how much of your donations will be eaten up by processing costs:
The most extreme case I've seen, from my days working at a nonprofit, was an elderly man who sent $3 checks to 75 charities. Since it costs more than that to process a donation, this poor guy was spending $225 to take money from his favorite organizations.
By contrast, at the high end you definitely need to divide your giving. If a someone decided to give $1B to the AMF it would definitely do a lot of good. Because charities have limited room for more funding, however, after the first $20M or so there are probably other anti-malaria organizations that could do more with the money. And at some point we beat malaria and so other interventions start having a greater impact for your money.
Most of us, however, are giving enough that our donations are well above the processing-cost level but not enough to satisfy an organization's room for more funding. So what do you do?
If one option is much better than another then you really do need to make the choice. The best ones are enough better than the average ones that you need to buckle down and pick the best.
But what about when you're not sure? Even after going through all the evidence you can find you just can't decide whether it's more effective to take the sure thing and help people now or support the extremely hard to evaluate but potentially crucial work of reducing the risk that our species wipes itself out. The strength of the economic argument for giving only to your top charity is proportional to the difference between it and your next choice. If the difference is small enough and you find it painful to pick only one it's just not worth it: give to both.
(It can also be worth it to give to multiple organizations because of what it indicates to other people. I help fund 80,000 Hours because I think spreading the idea of effective altruism is the most important thing I can do. But it looks kind of sketchy to only give to metacharities, so I divide my giving between them and GiveWell's top pick.)
I also posted this on my blog
Agree with purchasing non-sketchiness signalling and utilons separately. This is especially important if like jkaufman a lot of your value comes as an effective altruist role model
Agree that if diversification is the only way to get the elephant to part with its money then it might make sense.
Similarly, if you give all your donations to a single risky organization and they turn out to be incompetent then it might demotivate your future self. So you should hedge against this, which again can be done separately from purchasing the highest-expected-value thing.
Confused about what to do if we know we're in a situation where we're behaving far from rational agents but aren't sure exactly how. I think this is the case with purchasing xrisk reduction, and with failure to reach Aumann agreement between aspiring effective altruists. To what extent do the rules still apply?
Lots of valid reasons for diversification can also serve as handy rationalizations. Diversification feels like the right thing to do - and hey, here are the reasons why! I feel like diversification should feel like the wrong thing to do, and then possibly we should do it anyway but sort of grudgingly.