Baxter and Pratchett's first entry in this series, The Long Earth, proved a disappointment that didn't play to either author's strength. Despite that, it had some interesting moments. This book doesn't.
The Long War (the 'war' is a low key disagreement, entirely in the background), suggests that some collaborators just aren't a good match.,
The beginning is slow and jumbled, and frankly, it doesn't get much better. Characters have little connection to each other, and, worse, they're not interesting. Occasional tidbits of random information are thrown in for no good reason - for example, "At Watson Brake, Louisiana, five thousand years in the past, nomadic Native American hunter-gatherers had constructed major earthwork complexes," which has only the faintest of connections to the story. The disjointed nature of the writing continues throughout. One or both authors seems to feel compelled to connect every event to one or another popular song from recent decades. I like some of those songs too, but what's the point? It gets really wearing, not to mention dull. There are too many other examples of in jokes (that also aren't funny).
Characters are inconsistent too. Joshua is described as loth to leave his family, but seldom thinks about them. A young girl named Roberta is introduced as a precocious side character, but somehow morphs into a super-genius when we're not looking. Some of the science is weak - on a world dense with insects except at high altitudes, bats chase flies at the top of a mountain. Why? A world with few plants has too much carbon dioxide - from where? The trolls, who have a complex song-oriented culture of their own, seem to sing only human music.
Some parts of the story make very little sense. In particular, a late-book crisis feels entirely author-manufactured, and very unlikely to occur in real life. The resolution is apparently something to take on faith, since close inspection just leads to sputtering and cries of "What?" The putative big problem that drives Joshua just fades away, and suddenly all is well again. One couple fall in love for no real reason that I could see.
On a minor note, SFF has happily moved away (mostly) from the rampant sexism of earlier years. Occasional misandry, however, is apparently now okay, apparently. Nowhere near as widespread as the previous misogyny, but just as irritating. Here, it's confined mostly to one character, where it might be understood as character definition, were it not treated as cute.
Overall, this is just not an interesting book, and certainly not the interesting collaboration of two talented authors. Instead of building to something new, they've sunk to a lowest common denominator. Even at that, they just don't seem to have tried very hard. There were lots of more interesting things the authors could have done with this idea. Let contesting nations each have one world where they hold the land. Let each nation have one world where they're as large as they once used to be. Explore the trolls, or the elves, or the mysterious pyramid builders. Almost anything. Instead they've served up a jumbled mostly plot-less disappointment that's doesn't lead much of anywhere at all, using characters not even the authors seem to care about.
I had reservations about the first book, but gave this one a try. I don't recommend that you do. I won't be going on to the third or fourth; I just don't care what happens in them.