How would I write the first two chapters as one entire chapter using an objective 3rd person narrator?Since the chapters are all like diary entries...

To switch to a third person narrator means that you need
to change to a grammatiocal third person and use exposition, e.g. "As Shelter was on his
way to inspect ..."

The important aspect of shifting to a
third person narrator is deciding how to manage point of view. As Wayne Booth details in
his Rhetoric of Fiction, third person narrators can be either limited or omniscient, and
have distinct tones, voices, and attitudes. They can be impersonal tools of exposition
or have distinct voices, and they can be intrusive or
not.

Next, in switching to third person, think about how to
reorganize the material to be chronologically coherent. You need to create a timeline
that incorporates all the material from both chapters by figuring out their relative
chronologies and synthesizing them into a single
timeline.

X.J. Kennedy's Introduction to Fiction is a good
resource on how to write narrative exposition.