Was the second book in the Tanya, Shaar Yichud vEmunah ("The Gateway of Unity and Belief") originally published alongside the first book?
When the Tanya was first published in book form (1796), the first 2 sections were released. In a later printing (1806), the Baal HaTanya added a version of the 3rd section (it was later updated). The final two sections were added to the Tanya posthumously, by the Author's children. - Starting from about minute 4 of this lecture.
Prior to its publishing in book form it was released pamphlet form, but I'm not sure how that process went.
There is a "tradition handed down by chassidim that the Alter Rebbe originally intended to reverse the current order, with this second part of Tanya appearing first, as Part I, and the fifty-three chapters of the first part becoming Part II" - Lessons in Tanya, Intro to Shaar Hayichud VeHaEmunah