Lighting double sided surface

Hi peeps - "puki" here.

I've been messing about with double-sided surfaces. Lighting the thing as a single surface (regardless of which one) is okay. When I display both surfaces together, they won't light.

What am I missing?

Graphics3D 640,480,16,2 
SetBuffer BackBuffer() 

cam = CreateCamera() 
MoveEntity cam, 0,0,-12

light=CreateLight(2)
LightColor light,0,0,255
MoveEntity light,1000,1000,-1000

light2=CreateLight(2)
LightColor light2,255,0,0
MoveEntity light2,-1000,1000,-1000

mesh = CreateMesh() 

surface=CreateSurface(mesh)

triangle1 = CreateSurface(mesh)
v1 = AddVertex (triangle1,-2,-2,-2) 
v2 = AddVertex (triangle1, 0, 2, -2) 
v3 = AddVertex (triangle1, 2,-2,-2)
tri = AddTriangle (triangle1,v1,v2,v3)
tri = AddTriangle (triangle1,v3,v2,v1)

UpdateNormals mesh

While Not KeyHit(1)
TurnEntity mesh,0,1,0
RenderWorld
Flip 
Wend
End

Please help. I did try faffing about with it.

Thanks in advance.