multitexturing and vertex arrays

I can’t get the second texture unit work the way I want. What I want to do is put a basic texture in the first unnit (ex grass) and a sort of lightmap in the second. I thought the right way was to set the second tex as a luminance texture and then modulate with the first. The result is that the first tex is applied correctly while the lightmap seems to be “almost” not existent. I tried to activate only the second unit, but the result is the same. Can you help me? I’m working with arrays extensions, and I already checked the texture coords and they are ok.