Create "two-sided" curves I have been trying to create tubes for use in my game (in Unity). 

I've used this question to accomplish that. The extruded curve's normals are facing out, like I would expect. However, this means that when imported into Unity, the inside of the tube is invisible, and the collider that is generated is only recognizing the normals in the proper direction (like I would expect).

My solution so far has been to convert the curve to a mesh, duplicate it, and flip the normals. This works, but it feels super inefficient, and like I am missing some tool or modifier that might exist.

Any advice is appreciated, as it seems I am perpetually new to Blender :)
I think you are missing Solidify modifier.