1Mix cereals, 1 cup of the cheese crackers and 2 cups of the popcorn in large, microwavable bowl. Set aside.
2Place butter, brown sugar, corn syrup and salt in 2-cup microwavable measuring cup. Microwave on High 1 minute 30 seconds. Stir; microwave on High until bubbling. Remove from microwave; stir in baking soda. Mixture will foam up, but keep mixing until baking soda is dissolved. Pour over the cereal mixture; stir until fully coated.
3Microwave on High 4 to 5 minutes, stirring after every minute. Spread on waxed paper-lined baking sheet. Let cool for about 5 minutes to let caramel fully harden. Stir in remaining 1 cup cheese crackers and 1 cup popcorn.