How often do I give them food

We have two grown male cats who stay indoors. One is three years old & weighs around 18 lbs. He's not fat, just a big cat. The other is a year old & is too fat. They both eat from the same dish & only get dry cat food. We had gotten into the habit of putting food in their dish every time it was empty. However, the younger cat with the weight problem will eat any time there is food in the bowl, whether he's hungry or not. So I decided to only put food in the bowl at night. We've been doing this for a couple of weeks & he doesn't seem to be losing weight. I'm concerned that I may not be feeding them enough. Any recommendations would be greatly appreciated.

This is something you need a vet's advice about. Both as to whether they need to lose weight, and how to do it. Its dangerous to put your cat on a diet yourself, without a vets advice. Too rapid weight loss is just as dangerous as being over weight for cats.

Agreed, a vet should be consulted, also, my vet is famous for cautioning his clients- "You feed the cat, not the bowl"- Cats do not have large stomachs