I can’t imagine not finishing a pizza.
You and me, brother!
Cold pizza is great!
What if they offered their leftover split pea soup?!
Surely you are thinking of the guy who stops into a diner advertising pea soup as a special. He asks for the special, but is told the guy next to him got the last bowl. Sure enough, a bowl of pea soup is sitting at the next diner’s elbow, untouched!
After a while, the guy asks his neighbor, “Hey, if you aren’t gonna eat that soup, do you mind if I have it?” Other guy says, “Sure!” and passes it over.
First guy goes to town on the soup - and it is DELICIOUS! He gets about halfway through the bowl when he sees a dead rat lying at the bottom of the bowl. He can’t keep the soup down ad he pukes it back up right into the bowl. The diner next to him looks over and says, “Yep, that’s about as far as I got myself!”
I’ve heard the joke but wasn’t thinking of it. 
You would think the restaurant would not even allow it to happen if it could be helped. The anonymous couple offers you food that they say is o.k., they leave, you eat the food, something bad happens foodwise-you don’t know the couple, but you certainly know the name of the restaurant, and they can’t prove that they had nothing to do with whatever happened to the food. Sounds like a lawsuit waiting to happen, whether the two couples secretly know each other or not.
How about - what’s the difference between roast beef and pea soup?
Anyone can roast beef! 
I know we live in a litigious society, and anyone can sue anyone else anywhere for anything. But that’s not likely to be a winning case. As the plaintiff, you’re going to need to prove that the restaurant did something or breached some duty of care to recover. They don’t need to prove that they DIDN’T do anything to the food. That’s not how the legal system works.
Losing a suit costs a pretty plateful of money also, so why would a restaurant take that chance?
Take what chance? The premise is that the next table has not eaten part of their meal so they offer it to your table. But, surprise, they’re evil and have poisoned or tampered with the food, so you fall ill, or dead. So you sue the restaurant for what exactly? What are they meant to have done to prevent this?
You don’t know if the food was poisoned or if the food just made you sick, you don’t know who that couple was, so a lawyer might go after the known entity with the big pockets.
Again, what do you imagine the restaurant should do to prevent such a scenario?
Perhaps put up signs saying, “No sharing food”?
I think Czarcasm’s point is that you don’t know if it is the restaurant’s fault or the diners’ fault that you got sick. So both get sued.
A quesadilla doesnt come- always- in separate pieces, like pizza. And sure, I would eat my wifes leftovers, and vice versa.
I used to , but now I cant even finish a medium- with thinish crust
yes, but again, to win you actually need to prove that the restaurant did something affirmatively bad or failed to take action when it had a duty to. A plaintiff can’t win by just saying, “either the restaurant or this unknown person poisoned me, I have no clue which, but I want you to find the restaurant guilty.”
Another legal (or at least ethical) issue is that you’re sitting in a restaurant, eating food that you haven’t paid the restaurant for. You’re taking a table from paying customers.
Just being sued in the United States is expensive even if the opposing party has no case.
I doubt a judge would allow a case like this to proceed without the plaintiff demonstrating some theory of negligence on the part of the restaurant.
no doubt, but keep in mind that the plaintiffs are paying their attorney as well, and only the most desperate ambulance chaser is likely to take a case based on that type of scenario. edited to add that it’s also likely the restaurant has insurance that would be paying its legal fees in such a case.