The second texture's pixel colors will by multiplied with that weight, then added. I.e. 100+0.5*100 gives a resulting color of 150. If the weight is negative, like 100*-0.5*100, the second texture's color will be subtracted.
There is no other way then adding with this method (hence the name...). If you need more complex operations on the textures, you'll be better off with some image manipulation library. You can always create a Texture from an image.
BTW: Why are you writing in capital letters?