My wife and I sometime split meals as she eats like a bird. I also don't recall having any issues or extra charges to split. In fact, they are typically quite generous. When splitting a entire, I've found they will plate it with half the entree but full sides on both plates! (SciFi Drive-in at DHS has been especially nice about splitting meals).
And while tipping is a personal matter, please consider tipping based upon the price of two full meals as wait staff is providing that service.
Cheers, Steve