This still isn't enough to invalidate the argument against diversifying. I'm not fully convinced by it, but...

Suppose your money would be enough to increase charity A's chance of finding a cure from 50% to 50.08%, or charity B's chance from 50% to 50.06%, or by splitting the money increase A's to 50.04 and B's to 50.03, I'm pretty sure you're better off giving it all to A, which can increase the chance of at least one finding a cure from 75% to 75.04%.

Suppose both charities have diminishing returns so funding to increase chance beyond 55% is less effective. That's irrelevant to the situation where we aren't in that range.

Suppose charity A had a 30% of being either corrupt or taking a completely useless approach to the cure, in which case it wouldn't find it no matter how much money was donated. So long as the 50%, 50.04% and 50.08% figures have taken this into account, you don't need to consider it further.

Suppose one of the charities had already hit diminishing returns, and would have been able to increase from 1% chance of success to 2% with the amount of your donation, if they hadn't already had enough money for a 50% chance. That's irrelevant to the situation where we aren't in that range.

I only chose 50% to make the maths a bit easier, so long as neither is near 100% similar arguments apply, though you need to make sure you're considering each charity's effect on the total chance for a cure, not their chance of discovering it themselves.