Diet food for thin, heathy cat - Harmful?

HELP!!
I have a very fat cat who has been prescribed Hill's diet food to loose 30% of her weight.
The problem is our other cat is perfectly heathy and a nice weight for her small size. Will it harm her to eat the diet food. She has decided she prefers it!
Also how on earth do you feed 2 cats seperatly when they panic if they cannot see each other? These two cats are peas in a pod and will worry for each other when separated, which means they won't eat when stressed.

Any advice on either matter would be great.

(First time on this site and already finding it interesting, amusing and altogether friendy. In my favourites immediatly!)

Thanks

I would think it would be harmful for her, but she may have to eat more to get the same amount of calories she was getting with the regualr food.

Exactly my thoughts. However, there is nothing in the diet Hill's R/D that will actually harm her. It's just much lower in fat, and very high in fiber(to give a full feeling to reduce begging) The cat of ideal weight won't be getting enough calories & may end up underweight as a result. With this in mind, I wouldn't make it her permanent diet. She needs something more caloric. But, if she sneaks a few pieces of the r/d, there is absolutely no harm.

I feed all 4 of my cats in the same room. The all have individual bowls, and I just stand over them. It's a pain, but it works.

Nothing will harm the smaller cat from eating the low fat food, but in turn, is the overweight cat eating the non-diet food?

You can start to slowly introduce them to separate feedings. Feed them once in the morning and once in the evening.They are so hungry by that point, they will only eat what is placed in front of them.

Or as kris mentioned, you could feed them and just watch they don't eat each other's food.