We can modify the CSS as follows to create a circle with two borders:

div {
  width: 20em;
  height: 20em;
  border-radius: 50%;
  background-color: red;
  border: 4px solid #fff;
  box-shadow: 0 0 0 5px red;
}

This CSS adds a second border using the box-shadow property, which creates a 5px wide red shadow around the circle, effectively creating the illusion of a second border. The color of the second border is determined by the red value in the box-shadow property.

The provided CSS achieves the desired effect, creating a circle with two distinct borders that responds fluidly to changes in the container's size.