Free feeding adult cat food isn't going to help your fat cat get smaller.

I would try to feed them in different rooms for a while. Maybe your 'fat cat' won't be so fat if he's fed just what he needs and can't get into the kittens food.

I should talk! My cat Ghost is now at 16 lbs. I've switched from free feeding to twice a day with no treats, but I really don't know how much I should be feeding at each meal. I'm guessing at how much and letting him get his fill.

In a month I'll see if he's any lighter.

I wouldn't worry too much about WHAT but more about HOW MUCH each one gets. The kitten needs to get adequate nutrition.

Mary