Here’s the most relevant entry I found related to setting your board to a 2‑layer stackup:
“Multi-Layer PCB Design
Configuring Board Stack-up
New projects are configured with a 4-layer stack-up by default. Here's how to configure a different stack-up:
- Go to the "Layout" object in the "Objects" tab on the right-side panel of the PCB editor
- Select the "Layout" object
- In the right panel, add an "Object Specific rule" called "Stackup"
- Select a Stackup from the default list or click on the pencil icon to create a custom stackup with the Stackup Editor