Fat at 6 months is not good! You are feeding them way too much. At 6 months they don't need kitten food, are nearly at adult size, and they don't nearly as much as young kittens who are rapidly growing.
Many cats fed unlimited dry food do end up obese. Personally I would feed canned as well. It's more satisfying, plus they'll get a lot of water from it.
Buy them adult food and go according to the directions. If they don't lose weight, or get fatter, cut down the amounts.