This is a matter of history repeating itself. To Green Angel Tower, the final volume of Memory, Sorrow and Thorn, Williams' prior work in the same world and possibly the longest epic fantasy novel ever written (and around the fifth-longest SFF novel of all time), was split into two volumes in paperback. The final book in the Shadowmarch series was also split into two volumes for publication, Shadowrise and Shadowheart. Williams deliberately paced his Otherland series as four volumes, fearing the same thing would happen again, and very nearly extended that series to five books as the final volume came in so long, but they were able to keep it to four.
At the moment it is unclear if the two volumes will be published as "Part I" and "Part II" or if they'll have to be retitled. Tad is currently rewriting and editing The Navigator's Children with a view to Part I being published in 2022 and hopefully Part II six to twelve months later.
Williams' next novel, Brothers of the Wind, a prequel to the entire Osten Ard saga so far, will be published this November.