In other words, view the SI and FHI as one charity with different departments and your ability to contribute separately as a way to weight different sub-goals aimed at the same overall big problem, saving humanity.
Four!
"I'd rather live with a good question than a bad answer." -- Aryeh Frimer
I am not sure how to interpret your comment:
I'll comment on the first interpretation that I deem most likely.
To fix complex problems you have to solve many other problems at the same time, problems that are either directly relevant to the bigger problem or necessitated by other needs.
That the Singularity ...
Underground Q&A session with Nick Bostrom (http://www.nickbostrom.com) on existential risks and artificial intelligence with the Oxford Transhumanists (recorded 10 October 2011).
http://www.youtube.com/watch?v=KQeijCRJSog