Multitexturing question

You can't as that blending equation requires access to the frame buffer, and you don't have that in the texture combiners.

But assuming you mean you want to combine two texture the same way you combine fragmens with that blending function, you can set the first combiner to multiply the two textures, and the second combiner to multiply the two textures again and add to the previous result. Check out NV_texture_env_combine4 and/or ATI_texture_env_combine3 on how to setup the multiply and add in the second combiner.

edit: Bah, you can, of course, multiply them in the first combiner and set the RGB scale to 2, and then the last combiner to just pass throught the result from the previous combiner.