This is what I usually say - We ended up having a later lunch than planned, so do you mind if the two ladies split a meal, and the gentleman will have his own entree?
Because I know it's not allowed at Via Napoli, I always pay OOP there; in which case you can split a single serve pizza. (Two single serve pizzas for three people and you are still walking out of there with leftovers.) Other than that, not one single time have we been refused. And as stated by someone else, nine times of ten, the kitchen will neatly split it for you.
It's not cheating. At all. Your conscience should be entirely clear.