In my preperation for an exam I came across the following question:

In this question I treated the hinged laders as one body and thereby resulted forces and took moments. However, this does not yield the correct answer, and moreover, doesn't tell me which ladder slips first. The markscheme presents the following diagram

My question is, when can we treat a system as a single object? Does this only work only in the context of resolving forces but not with moments? If anybody could assist me in intituively thinking about forces, moments and such systems I would gretly appriciate it.