I've just been discussing this with someone who does a lot of club and social activities. He says the thing is to be positive about the fact that you are paying for one dinner rather than stressing the fact that you aren't paying for both (which probably no-one would expect anyway.) So you could say something about: "Two dinners, and the second on is on us!" or something that makes it clear you are paying for just the one. Hope you have a good time.