To create a horizontal slide use three dashes surrounded by two blank lines. To create vertical slides use two dashes surrounded by two blank lines:
# Slide 1
---
# Slide 2.1
--
# Slide 2.2
# Slide 1
---
# Slide 2.1
--
# Slide 2.2
Tip
It is possible to change the vertical / horizontal slide separator by changing the separator or verticalSeparator property in frontmatter.
See yaml frontmatter docs