I have not been to Via Napoli myself yet, but from what I've read you would have two options here:
1. Ordering half their signature pepperoni and half a white pizza should be possible, but just be aware that the tomato sauce from the pepperoni side will bleed into the white side to some extent.
2. Order a white pizza like the Quattro Formaggi and you should be able to pay to add pepperoni as an extra topping on either one half or the whole thing.