How do i keep my adult cat food and my kitten food separate?

I just got a new kitten, he is 10 weeks old. I have another cat, she is a year and a half old. They keep trying to eat each others food.. how do I stop this? They are really good about sharing a litter box, and spend a lot of time playing together. But I cant seem to stop them from sharing all of their food. Im not with them 24/7, so how do I know if my little kitten is getting all of the nutrition he needs from his kitten food?