I can picture them eating with each other a lot, but oddly enough not when camping. Which is a part of their plan. Ignis would put vegetables on Noctis’ plate and expect them to be eaten. So they’d sit across each other and wait until his back was turned and they were done eating what they wanted. For Noctis, he’d eat everything but the vegetables, and for his S/O, They’d try to leave something else for Noctis to eat in exchange for his vegetables.
Then his S/O would get up and walk in front of Noctis, swap plates with him in a fluid movement, and keep walking away with the veggies. It was easy and smooth and Ignis had yet to catch them. (They’d been doing this since they were young after all.)
Gladio would feel as if he was betraying Ignis a little, but the first time they did this, they had walked over to him and promptly threatened to kick his shins everyday, for the rest of his life, if he ratted them out. So he doesn’t say a word, and even gets a chuckle out of how practiced the two are.
Prompto on the other hand, they’d just have to shoot a look at and he’d turn the other way. He’d remember when they were in high school, and Noctis’ S/O sat on him and tickled him until he begged for mercy for a rather unsavory picture he once took of them. He did NOT want to cross them ever again.