I'm wondering the same (not specifically about financial support, but in general). My thought is that it would make sense to have the letter of explanation at the end of each document that needs explanation, instead of having it in a separate document.
Along the same lines, would it be better to combine documents that aim to fulfill a specific requirement of the checklist or leave them all separate? For instance, combining the proof of joint bank accounts with proof of joint utility bills (e.g. telephone, electricity, etc) to show that we are living at the same address.
Maybe I'm overthinking it, but what makes me wonder if IRCC prefers separate documents is one of the requirements in the checklist about photos that says the following: "Please upload a separate document providing a brief explanation of the context of each photo". I'd think that it would be easier for them to get a PDF with photos and the explanation under each of them instead of a separate document. But, apparently, they do want another file just with the explanation.
Sorry if these are dumb questions, but any insight would be appreciated.