Let me get this straight: Two islands, otherwise identical in all ways, each populated by different groups (differing only in ideology) of individuals carefully selected for the sole quality of being completely and totally incapable of thinking for themselves???
I forsee an awful lot of Starved~To~Death dumbasses.
Answer: Between 2 and 10 days without water, depending on how hot/cold it is. If there's plenty of water, then 4~6 weeks without food. Longer if they decide to amuse the rest of us by killing and eating each other.
{edit} If they *do* decide to kill and eat each other: Call the networks, put it on Pay Per View, and use the profits to feed 3rd world dictat*** (sorry - got sidetracked into the realities of Foreign Aid for a sec...) the starving masses.
You know - Because there's not enough irony in the world already.