Here's the post:
(generously saved and quoted by Jeremy)

MESSAGE: YOU _CAN_ STACK HIGHLIGHTS...!
AUTHOR: rene
DATE: 11/10/99, at 11:09 a.m. Response to: Re: Bump question
Author: Michael Vance
Date: 11/9/99, at 9:49 p.m.

"I have found one thing you can't do is stack highlight settings the
way you can others, which is a shame. It would be nice if a material
have more than one highlight property..."

"Hi, in fact there IS a (well - uh - dirty) trick to do it!

1) Create your bottommost material with whatever properties you may
     need (color, bump, transparency ...).

2) Apply to this material the "outermost highlight" properties (example:
     width 40%, strength - 30%); you may also want to include some
     highlight colour.

3) Create another material (the "upper" or second highlight material).

4) In this material JUST activate the highlight, genlock and, if desired,
     highlight color properties.

     Now the trick:

5) Load as genlock map some pic that entirely consists of a light* grey,
     made best experiences with about 5% to 10% grey. (You may have
     to use Photoshop or a similar prog to create the pic, it can be quite
     small)

6) Apply to the "upper" texture your desired second highlight properties,
     (IMPORTANT!!!), multiply the highlight strength with a factor that
     from 100 divided by your genlock-map´s greylevel percentage
     (example: pic= 10% grey, factor= 100/10% -> 10, desired highlight
     strength = 100% -> necessary highlight strenghth= 100% * 10 =
     1000%). The highlight strength can only be set to a max of 1000%,
     so if the highlight is desired to be 100% (pure white) the genlock map
     can´t lighter than 10% grey. )

7) Apply your textures (bottommost, then uppermost tex, don«t mix!)
     and render. It really works!
     There´s one great advantage, yet also a disadvantage coming with
     this technique.
     First one: You really can stack as much highlights to your objects as
     you want!!!
     Second one : As every genlock also does absorb some of the
     underlying texture´s properties (color, highlight...) you may need to
     get your bottom texture by some factor "lighter and brighter" than
     needed for the final result... For light textures with multi-highlights
     you may run into some difficulties to reach such a really _light_
     ground color - just play around a bit. There are no such probs with
     (only concerning their color) quite dark textures, polished steel,
     stone, rubber or the like...

I figured out this technique due to my mac´s humbling rendering capacities
- the bhodinut´s multi-highlight shaders do their job really well, but it´s such
a drag waiting for hours, just to get the pic... the described is FAST!"

my .02 :) rene

* IMHO it should be a "dark" grey!      Herbert Fahrnholz


That´s the text from the forum.
There is also the fact, that adjusting the genlock property you MUST activate
the option "Smooth" , because the trick doesn´t work otherwise.
By that unfortunately have to pass the GO- and SE-versions, which don´t have
this option at their disposal.

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