Double sided polygons have one single normal per vertex facing opposite directions.

[Gancian]

I'm not sure if you're already aware of the issue, however if a polygon of a dynamically lit object has two sides, each vertex inherits only one normal with alternate directions (one inside, one outside), resulting in messed up lighting. It doesn't matter if in 3D editing programs I manually correct the normals, they automatically get reshuffled on importing. The only solution rn is manually splitting the object into two meshes (but this doesn't bode well with Statics, which can only have one mesh) or using the conventional Tomb Editor prefixes to set textures as double sided (but this makes it impossible to apply different textures on two sides of a polygon)