The only thing you can do is to keep them separated during feeding. Keep one in the bathroom or somewhere while the other eats. Nothing else you can do. It won't hurt them to cry while the other is eating. Most people with a young animal and and older one go through this until the younger one can eat the adult food.
Also, Never, ever, squirt lemon in their mouths. They will associate this with thinking they are bad for eating, period.... They don't know the difference between foods, only you do! Separate them and then you don't need to be doing any disciplining.