What happens if you use more than one ability that splits incoming damage with another creature?

Question

3.5 includes multiple abilities that redirect a portion of incoming damage to other creatures. For instance:

The psionic power Share Pain states:

You take half damage from all attacks that deal hit point damage to you, and the subject takes the remainder. The amount of damage not taken by you is taken by the subject.

Similarly, the Shield Self ability granted by the vestige Dahlver-Nar (Tome of Magic, p. 27) states:

...you can designate one creature within 10 feet per effective binder level to share the damage you take. As long as the subject creature remains within range, you take only half damage from all effects that deal hit point damage, and it takes the rest.

If a creature is protected from damage by more than one such ability, what happens?

• Do the effects stack additively, granting de facto immunity to hit point damage (each ability redirecting half of the incoming damage; 50% + 50% = 100%)?
• Do they stack sequentially/multiplicatively (50% of damage redirected by the first ability, then 50% of the remaining damage redirected by the second, resulting in the protected creature taking 25% of the incoming damage)?
• Do they not stack at all?

Answer 1

This falls under the transparency rules of Same Effects.

Same Effect More than Once in Different Strengths:

In cases when two or more similar or identical effects are operating in the same area or on the same target, but at different strengths, only the best one applies. For example, a character under the influence of both the oak body power and the iron body spell benefits only from the stronger effect (in this case, iron body). If one power or spell is dispelled or its duration runs out, the other power or spell remains in effect (assuming its duration has not yet expired).

Same Effect with Differing Results:

The same power or spell can sometimes produce varying effects if applied to the same recipient more than once. For example, a shadow body power could turn a psion into a living shadow, but if it is immediately followed by metamorphosis, even while the shadow body would normally remain in effect, the effect of metamorphosis trumps the shadow body. If metamorphosis were followed by a series of polymorph spells cast by an interfering wizard, the last effect in the series trumps the others. None of the previous spells or powers are actually removed or dispelled, but their effects become irrelevant while the final spell or power in the series lasts.

Emphasis added. I realize that "same effect, same strength, multiple times" is not specifically called out in the rules cited, but I believe that these two rules combined still logically apply to this question's case. Notice how the examples are categorizing different spells together in "like effect" groups.

Therefore, in my opinion, the final effect in question being dominant, is the most balanced way to rule this. Thus, they would not stack.

Expanded Psionics Handbook, p56.