Create reveal.js stacks using quarto without level 1 headings

Question

Slide stacks are being created if level 1 headings are used for the horizontal axis and level 2 headings for the vertical axis:

---
title: "Presentation"
format:
  revealjs:
    navigation-mode: vertical
---

<!-- Stack 1 -->
# Slide 1

## Slide 1.1

## Slide 1.2

<!-- Stack 2 -->
# Slide 2

## Slide 2.1

## Slide 2.2

The above .qmd file renders the following html content:

<section class="stack present">
    <section id="slide-1" class="title-slide slide level1 center present">
        <h1>Slide 1</h1>
    </section>
    <section class="slide level2 future">
        <h2>Slide 1.1</h2>
    </section>
    <section class="slide level2 future">
        <h2>Slide 1.2</h2>
    </section></section>
</section>
...

Is it possible to create stacks using quarto without using level 1 headings?

<!-- Stack 1 -->
## Slide 1.1

## Slide 1.2

<!-- Stack 2 -->
## Slide 2.1

## Slide 2.2