Oh, right, sorry, you were talking about page count.
My parents have a copy of Les Miserables that's 1492 pages in a single volume. So, technically, it can be done somehow. There may be other technical considerations, though. The machines Tor's contracted printing house uses for binding may have a maximum size less than 1000 pages, for example, necessitating a switch to a more expensive printing house. There would be a point at which printing two books is more economical.
Ultimately, yes, it's a question of economics. And Harriet, with her publishing experience, may be more pragmatic in regards to the practical concerns of the publisher than Jim was and may have been planning to ultimately convince him to allow a split. And regardless of Jim's statements on the matter, it's difficult to say he never would have been convinced. However, this is all moot. Jim has gone, and the moral force with which he may have stood up to publishing demands can no longer be brought to bear. The book won't be split without Harriet's permission, that's for sure, but how she will weigh in on the matter is currently unknown.