Can The Forge be chopped into two 80K books?
Short answer is “yes.” Anything can be rewritten. I’ve spent decades writing (and rewriting and refactoring) software.
The more fundamental question is “Should The Forge be chopped into two books?” The Darkness and The Forge as separate books can provide some additional focus, tightening the narrative arc and focusing on The Mage’s path out of the darkness and toward the power of the forge. It decomposes the original journey into two sub-journeys, each with separate opponents.
Or.
Should I follow what sounds (to me) like advice from Brandon Sanderson and write another book (or two or three) also set in this world?
Other writers have talked about rejection as part of the process. I think I remember a blog post by Cory Doctorow suggesting that a creative is only as valuable as their next work in process.
See Randomness and Inspiration for some notes on my approach to world-building. Because of the scope of the planning, I have other interesting events that are part of the world’s timeline. Perhaps I need to set The Forge aside and look at some other character and other journeys?
The 137th Great Mage is calling to me. Unduly influenced by high mages; jaded and dissolute; confronted by lies: they're talked into searching for a lost artifact that could serve as a catalyst to remind the scattered people of their previous glories and reunite the Mage Lands. The results are grim and dark.