The age-old fantasy writer’s dilemma: how much of the world exists – both in your own head and more importantly, in the heads of your characters. If your setting is only medieval-level, it would strain credulity for all but the most exceptional navigator to have an accurate map of the entire globe (or flat sheet, or dome on the turtle’s back, etc.). But I work best starting zoomed-out and then zooming in as needed. And with a world like Bifrons that has such varied landscape depending on location, you need to see the bigger picture.
The human inhabitant of Bifrons – no, there’s no lost colony, they just happened to evolve there in a masterwork of convergent evolution! – are at a roughly 18th-19th level of tech. So it’s not unreasonable they have mapped the majority of the Dayside. The Nightside is a lot harder to explore, but I’ve handwaved it a little by deciding that the “educated guesses” by the world’s best geographers tend to be uncannily good. A shame they won’t recognize it for another several hundred of our years.