This isn't space opera (as certain adulatory reviewers call it), it's space narcissism.
The two main characters, Red and Blue, are opposing superagents in a time war, traveling wherever and whenever they wish, manipulating and destroying people, cities, even civilizations, to further their side's agenda. They begin writing letters back and forth to each other as sort of a masturbatory exercise in mutual approval, then fall in love.
It's Griffin and Sabine: An Extraordinary Correspondence with a time travel plot wrapper.
Remember: the garden-variety narcissist is the star of his or her own movie: everyone else is an extra. And this book takes modern narcissism to new heights: While Red and Blue enjoy the drug of attention from each other, not just other people but entire civilizations are their extras. Worse, this reader found himself totally indifferent to the fates of these two characters.
Finally and worst of all, the authors seem indifferent to their own story and don't even bother to resolve their own plot. Perhaps it's just another extra too.