My kitten and senior cat are eating the wrong foods, is that ok?

We have a 10 year old cat and we just got a kitten a couple weeks ago. I have kitten food in one room and senior food in another room. They both eat their own food and eachothers food. My senior cat is overweight. I don't know how to get them to eat only their own food. They don't eat on command and munch on it throughout the day. Any advice? I hope the kitten food won't make my cat fatter!