One thing you could try is to draw the border as a line with a thickness first, then draw the inner part of the circle as a filled shape;
shapeRenderer.circle(x, y, radius + borderWidth);
shapeRenderer.circle(x, y, radius);
You might have to query for the capabilities for the line width using
glGet and asking for 'GL_ALIASED_LINE_WIDTH_RANGE' and 'GL_SMOOTH_LINE_WIDTH_RANGE' (opengl reference), and it will possibly make the drawing device dependent.
If that does not work, you could try render to texture first, using a frame buffer with no blending, then draw that buffer (with blending) to screen;
// Position and size of circle, and border
float circleX = 100;
float circleY = 100;
// Total size of the buffer
int size = (int)((radius+border)*2);
// Create the buffer and get a region to it
FrameBuffer buffer = new FrameBuffer(Format.RGBA8888, size, size, false);
TextureRegion region = new TextureRegion(buffer.getColorBufferTexture());
// Begin the buffer, any draw calls made after this affect the buffer, not the "screen"
ShapeRenderer renderer = new ShapeRenderer();
// Draw border, centered on buffer
renderer.setColor(new Color(1, 0, 0, 1.0f)
renderer.circle(size / 2, size / 2, radius + border);
// Draw center, centered on buffer
renderer.setColor(new Color(1, 1, 1, 0.5f)
renderer.circle(size / 2, size / 2, radius);
renderer.end(); // shapes done
buffer.end(); // buffer done
// Now render the buffer onto the "screen"
SpriteBatch batch = new SpriteBatch();
// Move to the buffer offset representing the circle's position
batch.draw(region, circleX - size, circleY - size);
Obviously, in the example above, try to new up the
renderer outside of your render method so that you don't pay the performance penalty of doing that every time.