For fairly obvious reasons, this book reminded me of Kate Atkinson's Life After Life. This book was rather tidier than Atkinson's and its conceit was a little less overwhelming. This book was more restrained - two stories, two worlds, two sets of possibilities and two people. It felt like a tighter narrative and it lacked the...pointedness of Atkinson's work.
Mild spoilers for the end to follow.
Having said all that, I liked Life After Life better. I wanted resolution in this book or, failing that, a return to the mystery. What I got was what felt like platitudes. I'm young enough to find those irritating. I either wanted MEANING or I wanted meaning denied. I didn't want to be told it didn't matter. So the book failed to sell me on its resolution, which made all the earlier, well-written and interesting bits more difficult to work with since they never went anywhere. I enjoyed this book and so much of it really did work - especially the two different ways that the world works out. I'm definitely on board with the book's politics. And yet...it was missing something.
What I found so interesting, about both this book and Atkinson's, was that the hardest parts to read were the bits that dealt with bad marriages and mistreatment by family members. Maybe it's a question of scale - giant catastrophes seem unstoppable by humans, but individual lives feel like the sorts of things we should be able to make better.