I've read somewhere in the Internet (in a Divine Metamagic build) that having levels as Cleric and Paladin lets you have twice thee turn undead attempts per day.
Is that true?
Shockingly, the rules specifically address this. Looking at the rules for multiclass characters:
I'd think that this makes it pretty clear that you get the ability only once (and thus have one set of turning attempts), but add your levels together to find your effective turning level.
In case you think this is ambiguous, the wording about familiars is much the same:
Here it is clear that the words "have the same ability" are intended to mean that you are only granted the ability once. Otherwise a sorcerer/wizard would have two familiars, not the one implied by the second sentence.