Goodreads Librarians Group discussion

Hi, I'm unsure which publication date should be considered the original in this case.

The book was first published as one volume, then later republished in two split volumes (the rest of the series also follows the split-volume format). Should the split editions have their publication date set as the original, or should full book publication date set as original also for split edition?

(For context, the book currently has only one full edition and one split edition. In the future, it will likely also be published as an ebook and audiobook.)

The original publication date is the date that the book was first published. In the case you cite, that is the book in one volume. As you noted, the split volumes are a republication of the original volume.