They Both Die at the End by Adam Silvera

Tracy says:
Starts off with a quiet, heart-pounding tension and doesn’t let go till the end. The two narrators are the perfect voices to tell a story about dying too young—and knowing it’s coming. I didn’t want to put this book down. Yes, the title tells us how it will all end (maybe… probably!), but the journey and the question of how they would die and how these two would connect along the way kept me completely hooked.