This is a pretty common issue. It’s like both your cat’s read an 80s movie script, so automatically there needs to be one fat kid who’s personality trait is to be preoccupied with food.
This will require some creativity.
The classic trick is that if you have one biggie smalls cat who can’t jump high and one skinny minnie, then you just elevate skinny minnie’s food high so fatty can’t get it.
Sometimes you have to feed them in separate areas, which means if they were free fed before, they shouldn't be now.
Sometimes, they will eat the same food, but the skinny cat gets extra in a separate area.
Or, you can try a whole food approach. You choosing a diet, or combining diets so the effect is averaged out - the fat cat stabilizes, and doesn't gain any more weight, and the skinny kitty stabilizes and doesn't lose any.
People are relieved when I tell them that sometimes success is just not gaining anymore weight. That is an admirable goal in and of itself.