If you're using normals on the material its best to duplicated and flip anyway, otherwise one side will get lit incorrectly. edit: actually anytime you do this it will get lit incorreclty on one side, just with normals its usally more apparent.
In the Material Editor > main material properties > check: "Two Sided" you'll probably end up creating some polys for the inside because they'll share lighting. However the outside is lit, that is how the inside will be lit, unless you actually add geometry to the inside.