low-poly

Whether or not you have 'backface culling' active will determine if you can see the surface through the opposite side of the polygon, but it is still only a single surface with one set of UVs which will only be lit by frontal side (the back side texture is just flipped and uses the same lighting as the front). If you can stick your camera inside of your model and still see the inverted insides, backface culling is off; if your model becomes invisible when you put your camera inside of it, then backface culling is active.

If you would like a plane with two texturable sides that is compatible with lighting from both sides and their own separate UV areas, copy and flip the plane and then combine the two together into a single object (but do NOT 'weld' the vertices together as this would make both quads share the same vertices which will end in one of several disasters depending on your modeling package). Backface culling must be enabled for this to work or you will get z-fighting from the planes perfectly overlapping. Effectively you end up with two separate quads, two sets of 4 vertices in pairs that are in the exact same positions, but with faces opposing each other, which can be textured and lit as usual. It is hacky, but it will work.

If this is confusing let me know and I can give you a tangible example to work with.