It is automatically a Parameter in your Constant Material Instances. You can’t change it at runtime.
Create a Material Instance out of your One-sided Material. Under Material Property Override you can override the Two-Sided Property.
You can then create a Dynamic Material Instance out of this Material Instance.
As I mentioned, you can’t set the Two-Sided at runtime! You have to change the Material (/-Instance) if you want to switch between Two and One sided
