In this case, a decision to donate 100% of your money to SIAI would mean that all those people implementing a decision process sufficiently similar to your own would donate 100% of their money to SIAI. A decision to donate 50% of your money to SIAI and 50% to Charity Option B would imply a similar split for all those people as well.

This argument assumes that the people using a similar decision process are faced with the same evidence. In particular, if they made their decision significantly later then they would know about your donation (not directly, but if SIAI now had significantly more funds they could know about it).

If all decision makers were perfectly rational and omniscient, but didn't have to make their decisions at the same time, then you wouldn't expect to see the 50/50 splitting. You would expect everyone to donate to the charity for which the current marginal usefulness is greatest. In the situation you envision, the marginal usefulness would decrease over time, until eventually donors would notice that it was no longer the best option, and then start diverting their funding. Perhaps once this sort of equilibrium is reached splitting your money is advisable, but we are extremely unlikely to be anywhere near such an equilibrium (with respect to my personal values) unless there is an explicit mechanism pushing us towards it. This would probably require postulating a lot of brilliant rational donors with identical values.