What's the most brokenly strong character you have ever made? (Just for yucks)

[Elkad]

What do you do with a Cleric with two domains that grant alternate forms of turning (elemental domains, say, or the Warforged domain)? Do you do three pools, or does this Cleric-10 now Rebuke as a Cleric-30?

Neither. 2 domains just adds to what he can turn. He doesn't get any more attempts in his pool. And he's still a Cleric10 and turns as one.
If he has multiple classes with turning, then the class levels would be additive. But I don't add for each creature type.