They both die at the end (alternate ending)

Mateo

I wake up feeling invincible. I don't check the time because I don't want anything to shatter my survivor spirit. In my head, I'm already in another day. I have beat Death-Cast's prediction, the first person in history to do so. I put my glasses back on, kiss Rufus's forehead, and watch him resting. Nervous, I reach for his heart, and I'm relieved it is still beating: he's invincible too.

I climb over Rufus and I bet he would kill me himself if he caught me leaving our safe island, but I want to introduce him to Dad. I leave the room and go to the kitchen to prepare tea for us. I set the pot over the stove's burner and check the cabinets for tea selections and decide on peppermint.

Before I switch on the burner, I remembered that Sean might not have fixed the burner if he read the letter. I went back to bed hoping I would die in my sleep.