It does the same thing if you just texture the plains. They look correct because culling is off by default on plains. If you turn culling on and rotate it so you can see it then you can see that the texture is mirrored and the normals are inversed.
autocam off
position camera 0, 0, -30
load image "grass2b.png", 1
make object plain 1, 20, 20
texture object 1, 1
make object plain 2, 20, 20
texture object 2, 1
set object cull 2, 1
position object 1, -10, 0, 0
position object 2, 10, 0, 0
xrotate object 2, 180
wait key
Gives the included result. Also yrotating a plain to 180 has no effect on it
Ah you guys new they were wrong as you made culling off by default and made the default y rotation be 180 to make them look correct. Run this code to see how it is really off. It turns culling on and sets all rotations to 0.
autocam off
position camera 0, 0, -30
load image "grass2b.png", 1
make object plain 1, 20, 20
texture object 1, 1
make object plain 2, 20, 20
texture object 2, 1
set object cull 2, 1
position object 1, -10, 0, 0
position object 2, 10, 0, 0
rotate object 2, 0, 0, 0
wait key