I'd also caution against PeteAtoms advice. If the two girls are in the same group of friends and they start talking about how you are obviously more interested in the hotter girl, then that just makes you look like a jerk and makes the less-hot girl feel awful about herself. Have a good time with the first girl, maybe you'll hit it off. If not, pursue the hotter girl after, if you can.
Yea, I wasn't really aware of the full picture at that point. But I'm not sure how much weight a graduation
dinner has, as far as romance is concerned. I mean, even with dances, my group of pals and gals never really got too hung up about who one went with because going with someone as just a friend was typical. Romantic interest may or may not even come into it. We would ride to the dance together, but everyone took turns dancing with everyone, even the boyfriends and girlfriends didn't dance with solely their partner. Everyone just had fun

But yeah, in my mind a dinner seems a lot less of a thing than going to a dance

Or is there dancing also involved at dinner? I don't know, lol, we never had any as I recall.
It all depends on how the first girl imagines the dinner going, OP.