I can certainly agree with your decision to do it your way, and creating the 'definitive wrk' is no easy undertaking. The only comment I can add is that i have zero interest in what the standard gauge did/does with their snow equipment, and I suspect there are an even greater number that are only interested in that standard gauge side and the barrow gauge is just a footnote to them. If it makes economic sense to create two books that are less costly than one master tome, I'd be more inclined to purchase one. For those dedicated dual gaugers, they can add both books to their collection.
No intention to do more than have you consider a pair of books - definitely all your show, and whatever you decide we will all be grateful for your contribution to the history of railroading in the West.