Does the Celestial Brilliance spell stack with itself?

Question

I've a couple of questions regarding stacking multiple area effect spells:

1. Does Celestial Brilliance spell (BoED p94) stack with itself? That is, if two instances of this spell are cast and an evil outsider is within both areas of effect, does it take 4d6 points of damage or 2d6? Why? (there are plenty of spells like that, e.g. wall of fire, it's just Celestial Brilliance which is easy to stack)

2. What if it was an emanation, instead of an unspecified area effect?

3. What if it was a spread?

Answer 1

They would not stack

It's covered by this sentence:

In most cases, modifiers to a given check or roll stack (combine for a cumulative effect) if they come from different sources and have different types (or no type at all), but do not stack if they have the same type or come from the same source (such as the same spell cast twice in succession). If the modifiers to a particular roll do not stack, only the best bonus and worst penalty applies. Dodge bonuses and circumstance bonuses however, do stack with one another unless otherwise specified.

Emphasis mine.

Note, that this is referring to Bonuses, but I believe the same applies to Damage as well, unless the spell says it stacks, it wouldn't stack.

Similar Reference in the Magic Section:

In cases when two or more identical spells are operating in the same area or on the same target, but at different strengths, only the best one applies.

Which I think covers the damage stacking aspect.

Answer 2

Damage stacks. Bonuses and penalties or similar usually do not: SRD on Casting Spells

STACKING EFFECTS
Spells that provide bonuses or penalties on attack rolls, damage rolls, saving throws, and other attributes usually do not stack with themselves. More generally, two bonuses of the same type don’t stack even if they come from different spells (or from effects other than spells; see Bonus Types, above).

Different Bonus Names : The bonuses or penalties from two different spells stack if the modifiers are of different types. A bonus that isn’t named stacks with any bonus.

Same Effect More than Once in Different Strengths : In cases when two or more identical spells are operating in the same area or on the same target, but at different strengths, only the best one applies.

Same Effect with Differing Results : The same spell can sometimes produce varying effects if applied to the same recipient more than once. Usually the last spell in the series trumps the others. None of the previous spells are actually removed or dispelled, but their effects become irrelevant while the final spell in the series lasts.

It's pretty clear that whole section deals with bonuses and penalties and not pure damage (even if they affect damage rolls of other attacks, they are not damage spells themselves).
It's also pretty neat for instance to imagine an enemy sandwiched between two Walls of Fire and a Blistering Radiance (or two) cast on top of him. They all do fire damage so same effect, but they should all apply damage normally. There's no reason not to stack.

I'll go even further and say that overlapping area spells do stack all their effects. What I mean is if an area has 2 spells that Slow entering creatures and a creature enters, he has to make two saves, one against each spell. If he fails both saves he still only gets movement halved, not quartered. The Slow effect on him is not stacked. However if dispel is cast on him it has to dispel both spells/effects to "cure" him. This view is supported by logic of Same Effect with Differing Results ("None of the previous spells are actually removed or dispelled") and the section on Charm stacking:

Multiple Mental Control Effects : Sometimes magical effects that establish mental control render each other irrelevant, such as a spell that removes the subjects ability to act. Mental controls that don’t remove the recipient’s ability to act usually do not interfere with each other. If a creature is under the mental control of two or more creatures, it tends to obey each to the best of its ability, and to the extent of the control each effect allows. If the controlled creature receives conflicting orders simultaneously, the competing controllers must make opposed Charisma checks to determine which one the creature obeys.