Several Passes for Terrain

Hi all;
I wanna make the following steps, considering a 2 tex units GPU:
1- render the base terrain (ex. rocks) texture with lightmap
2- render a layer (ex. grass), modulated by an attenuation texture;
3 - render aditional layers (ex snow…), modulated by attenuation textures.

I dont know how to configure the tex env combiners to achieve such effect...i wanna make it appear like the Torque terrain system.....thanks