1. Create a geometry, for example a tube. Copy the tube node inside the geometry node.
2. Create another geometry, delete the “file” node in it, paste the tube node here. Adjust the tube's position to make it mirror with the previous tube on YZ plane.
3. Create a bone on one tube.
4. Mirror the bone on YZ plane. (hit tab and type mirror to use the mirror tool)
5. Capture both tubes' geometry for their bone
6. Export whole thing (obj) as FBX.
7. Import it to Unity 5.
Then you will see one side of the tube's normal is flipped over, so it appears to be transparent.
