My own vote is for a single book, for the following reasons.
1) Precedence with the FRCS and the PSCS as well as other works. This is as far as my brain looks at it - a setting book - so that precedence and organizational structure feels more natural to me. I suspect it will feel that way to a lot of people, and given our ability to present in chapter forms which are faster to download, smaller to print and smaller to read online, I see no reason not to keep it a singular book. While there's an argument that two books makes for less printing I'll point out that most people - if they want a hardcopy of the release - are going to print both out anyway. So they're using just as much paper and ink either way (possibly more if the DMG repeats material in the PHB).
2) Because it is a setting book, determining what goes into the Ortho Players as opposed to the Ortho DMs is very difficult, if not downright arbitrary. If we go the arbitrary route then that makes it more difficult to find the material you're looking for swiftly because it could be in one book or the other. If we split the material based on relevance to the DM then there's just not enough DM exclusive material in a setting guide without quickly reaching the conclusion that we'll have a slender manual next to a thick tomb of doom. In which case we might as well have one book.
3) Related to the division of material from #2 - the editing and writing restrictions of splitting material for two books will make it very difficult for me to re-edit our current material. The writing already done is definitely in a single-book style, so it will easily double or triple the time it takes to go through editing as I will need to consider nearly paragraph sentence by sentence to see which book to move something to and hope I can reedit it afterwards for coherency. If we choose to have the authors do the rewrite instead - that will still take time, and editing time to place the materials correctly with the right introductions. And that's not even accounting for the authors who are no longer with us. As hard as it can be for us to get time in this sort of project to do the 'tedious' work - I just don't want to add to that end of the job. It took the PSCS over 5 years to get even close to done.
4) My last concern is a worry that the style of writing required for a 2 book release will stifle future writings both for the Ortho release and for future setting releases. A writer will have to keep the end goal in mind, and may find themselves tediously repeating material for one book that was already clearly laid out in the other in order to explain one subtle DM related difference. It can be a pain in the butt to write that way and I don't want to put any more stumbling blocks in front of volunteer writers than there already are as an online-no-profit project.