Yes! It is entirely possible to use both the first and second-level headings as markers for slides. This can be helpful when you only need to create a few slides and you don't need any additional subheadings for your presentation.
To change this behavior, all you need to do is to go to your document's preamble and look for #+OPTIONS:. This is a special variable that determines how your Org document will look and behave.
From there, you then need to find the H: parameter in the variable's value list. This is what controls which headings are converted to slides in Beamer. The corresponding value will dictate the level of heading that Org will use for its slides. In order to use the higher level headings, you need to provide a value of either 1 or 2.