If the answer is yes, then:
1. The character is subjected to an attack that deals 15 points of damage.
2. The character only takes half of this damage - 7 points.
3. The amount of damage not taken by the character - 8 points - is taken by Caster A and Caster B.
So a total of 23 points of damage results from the attack: 7 + 8 + 8.
But I suspect one of these two rules will apply:
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.
So if the two spells have differing caster levels, only the strongest will apply; alternatively, the last spell cast applies, while the first becomes irrelevant.
-Hyp.