Three.js self transparency, object should not be see through - three.js

Question

Codepen demonstrating the problem
https://codepen.io/anon/pen/GjJpYw?editors=0010
I have 2 meshes, one which contains 2 cubes, and the other which is 1 cube. The mesh with 2 cubes sandwiches the mesh with one cube (so the single cube is in the center). When I set all cubes to transparent but set the opacity of the center cube to 1, I would not expect to be able to see the back cube when looking through the front cube but I can.
I was wondering is there any easy way to fix this? This is a very simplified version of the problem I'm facing so I can't easily split the geometries. I also cannot just set transparent to false since ideally I would like to be able to have the middle cube partially transparent as well. Any suggestions?
var width = window.innerWidth;
var height = window.innerHeight;
var renderer = new THREE.WebGLRenderer({ antialias: true });
renderer.setSize(width, height);
document.body.appendChild(renderer.domElement);
var scene = new THREE.Scene();
var cubeGeometry = new THREE.CubeGeometry(100, 100, 100);
var cube = new THREE.Mesh(cubeGeometry);
cube.position.set(0, 25, -200);
var cube2 = new THREE.Mesh(cubeGeometry);
cube2.position.set(0, -25, 200);
cube.updateMatrix();
cube2.updateMatrix();
var singleGeometry = new THREE.Geometry();
singleGeometry.merge(cube.geometry, cube.matrix);
singleGeometry.merge(cube2.geometry, cube2.matrix);
var combinedMaterial = new THREE.MeshBasicMaterial({ color: 0xff0000, opacity: 0.5, transparent: true});
var mesh = new THREE.Mesh(singleGeometry, combinedMaterial);
var cubeGeometry = new THREE.CubeGeometry(200, 200, 200);
var cubeMaterial = new THREE.MeshBasicMaterial({ color: 0x0000ff, opacity: 0.8, transparent: true});
var cube = new THREE.Mesh(cubeGeometry, cubeMaterial);
scene.add(cube);
scene.add(mesh);
var camera = new THREE.PerspectiveCamera(60, width / height, 1, 1000);
camera.position.z = 500;
var controls = new THREE.OrbitControls(camera);
controls.addEventListener('change', render);
var pointLight = new THREE.PointLight(0xffffff);
pointLight.position.set(0, 300, 200);
scene.add(pointLight);
render();
animate();
function animate() {
requestAnimationFrame( animate );
controls.update();
}
function render() {
renderer.render( scene, camera );
}