Here’s one approach…
markdown:
+++
title = "Test"
date = 2020-08-22T07:02:03-04:00
draft = false
+++
This is the content for the **first** section.
<!-- section break -->
This is the content for the **second** section.
layouts/_default/single.html:
{{ define "main" }}
<h1>{{ .Title }}</h1>
{{- $sectionDelimiter := "<!-- section break -->" -}}
{{- $rawContentSections := split .RawContent $sectionDelimiter -}}
{{- if gt (len $rawContentSections) 1 -}}
<div id="section-1">
{{ index $rawContentSections 0 | .RenderString }}
</div>
<div id="section-2">
{{ index $rawContentSections 1 | .RenderString }}
</div>
{{- else -}}
<div id="content-not-split-into-sections">
{{ .Content }}
</div>
{{- end -}}
{{ end }}