Why are you sure that for you, $800 worth of food is less than 2 x utility of eating $400 worth of food? How would you convince somebody who thinks you are wrong (e.g. deluding yourself, or not telling the truth)?
It doesn't seem like it would apply for all instances and combinations of food. For me, I've had a $90 porterhouse, which was truly delicious, and was indeed worth more (not less) than if I had gotten one $45 alternative cut to eat then and a second one to take home. But extrapolated out for other food / time scales / diet preferences, buying two 5 pound packs of bacon for $40 gives me a lot more utility than $80 worth of pepperoni bits which are quite a bit more expensive per pound but very convenient as a ready-to-eat snack.
You could ask them to buy twice as much food as they normally would every time they go to a restaurant or the grocery store. They can either eat until they feel uncomfortable and gain weight or they can start throwing a ton of food away, either way the utility per dollar spent goes way down past a certain point.
Note that this assumes that you are already able to spend an optimal amount on food.
You're assuming that 'food' is a single fungible commodity. Spending twice as much on food doesn't mean buying twice as much food, it could also mean buying better quality food and going to higher quality restaurants. I could easily both double and triple my current food spending without eating more or wasting food.
But taste/enjoyment isn't the only purpose of food. You're doubling enjoyment, but not doubling the utility.
If I gave you the choice of only eating $1 of Cup-Noodles every day for 30 days ($30 total), or one single $300 meal at a high class restaurant, and nothing for 29 days, which is the smarter choice?
In America, it is actually surprisingly difficult to buy 'half as much food' at many places. Most mid-scale (Approx 10-30 USD/plate) restaurants attempt to convince you the service is worth the price by giving you far too much food (and also not paying their employees enough either; hence tips, which BTW I'd be inclined to stop paying in 'living minimum wage' cities (15+/hr)).
