linearity in composition


I’ve a scene in blender which I render in yafray with a texture spheremap as the world.
Those spheremaps contains different softboxes emulations.
I want to be able to composite all the generated image from the different softboxes, and once I’ve all spheremap ratios to achieve the desired effect, composite the spheremaps into a unique spheremaps to produce the final image using the composited spheremaps, rather than the composited images coming from different spheremaps.

So my question (as you might guess) is : is it linear ?
e.g. 10% blend of image with spheremap1 applied to image with spheremap2 = image with 10% blend of spheremap1 applied to spheremap2 ?