I have held fast to this since i started watching and reading Naruto... Although, I dont think they both die, just Naruto.. I think that Naruto sacrifices himself so that Sasuke will know that Naruto understood him and loved him like a brother. As I play this out in my head, I'm begining to think more and more that Maybe the unknown power that Itachi gave to Naruto was his essence of life...
As we know, Itachi, during his battle with Sasuke was weakened by some illness that has yet to be named or explained. Yet before his fight, he gave Naruto some of his power.. If it was per-say his lifeforce, that would explain his weakened state, and also why he told Naruto that "I hope you never have to use this.." He would not want Naruto to die, being the only one that he feels can save his brother, and be the brother that Itachi was denied. Itachi was Extremely intelligent, and could foresee many things and perhaps this as well.. So upon Naruto's death this power may be there to give him life again.. Kinda like the old lady did for Garra.. Itachi not having a backup plan? not likely.. he probably had 10 backup plans...
This of course is all theory and speculation.. However it is logical and possible..
Any thoughts?