graduate school - Can I accept two federal fellowships simultaneously and defer one?

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?