And… immediately after posting I found the solution…
So for my example above becomes:
<div class="sbs-container">
<div class="sbs-item">
{{ partial (.Get 0) }}
</div>
<div class="sbs-item">
{{.Inner | markdownify }}
</div>
</div>
and call using {{<…>}}
{{< side_by_side "stl.html" >}}
## Headding
Followed by paragraphs of text
{{< /side_by_side >}}
Edit:
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