(I seem to recall reading about word count *cuts* of that order of magnitude in the writing-advice books I picked up from used book sales at every opportunity growing up, so I'd be careful about making any firm decisions yet if you're still drafting it.)
The best recent example that leaps to mind is, unfortunately, one that I'm hesitant to recommend in this forum because it's (in addition to a fantastically-well-constructed fantasy world) a romance in the modern sense of that term: 
The Sharing Knife by Lois McMaster Bujold. The first two volumes of the series were designed to be a novel in two volumes; the third and fourth were originally intended to be the same, but turned out to be two one-volume novels. But here's some of the story-construction lessons that I think can be learned from those:
Try to end the first volume (and each non-final volume ...) with 
some sort of resolution, but pace it differently than you would the climax and resolution of a one-volume story.
It's helpful to slip some reminders of the world and of the-story-so-far into the beginning of the second (and following, if any) volumes, if you can do this subtly and not drag the pace of the story down, just in case a reader either hasn't read the first volume in a while or somehow hasn't read the first volume at all.
If possible, try to pick a natural dividing point between the two volumes. In 
The Sharing Knife, it's just after Our Heroes have gotten married and left her family's lands (and culture) to go visit his people. In 
The Lord of the Rings, the first Book (note that not only did the publisher split it into three volumes, Tolkien divided it into six Books) ends with the cliffhanger at the river, then the second picks up with his waking up in Rivendell, and 
Fellowship ends with the Breaking of the Fellowship.
Hope this helps; knowing my habits, I'll probably have to confront this sooner or later, but I'm nowhere near this point yet. 

(By the way, if you do split it, will you publish one volume first, then the second some months later, or both at once?)