I recently received both an NSF and an NDSEG fellowship. I see that I am not allowed to accept funding from both fellowships, but I wonder if it is possible to do something like: accept both, put the NSF on reserve for two years and forfeit the next year, then accept the last two years of funding from the NSF after the NDSEG has expired. Can someone confirm whether this is possible or not?
Answer: No, it is not possible. I called the NSF office concerning this issue, and they told me that I have to decline one of the fellowships. The same is true of any two federal fellowships: you can only accept one, period. This page also says the same thing, addressing the specific proposal mentioned in the question.