How do I make my game render both sides of a mesh?

I made a mesh of a jacket for a character in Blender 2.4 and imported it into Unity. Everything works fine but it would be better if the game would render both sides of the mesh.

There are parts of the baggy jacket mesh were the player should see some of the undershirt and then shadow from the space inside the jacket. Instead I see the game not even rendering the inner part of the jacket as if parts of it were 100% transparent. I know its because the normals are pointing the other way (outward) but there must be some way to make it render double-sidded.

I guess what Im asking is how do I render both the normals and 'flipped normals'?

Generally speaking, you don't (you can modify the shader to do it, but then the lighting on both sides is the same, which you don't want). Model both sides the polygons you want, duplicate, then flip normals.

You're looking for the Cull Off. That's a quick fix for the transparency thing, but ideally, like Eric said, you should actually model the exposed areas of the shirt. Turning off culling is a drastic performance hit.

P.S. Turning off the cull won't actually flip the normals for lighting calculations so it'll still be odd.