PREP: Preheat oven to 350. Line a 15x21-inch baking sheet (or 2 smaller baking sheets) with foil and grease with nonstick cooking spray.
CEREAL: Add 4 cups Rice Chex, 4 cups Corn Chex, and 1 cup mixed nuts to the prepared sheet pan.
SYRUP: In a heavy medium-sized pot, add the 1/2 cup butter (cut into individual tablespoons), 1 cup packed light brown sugar, and 1/4 cup corn syrup. Place over medium to medium-high heat and stir slowly but constantly, until butter is melted. Once melted, continue to stir until the mixture comes to a rapid boil. Boil for exactly 1 minute, stirring constantly, and remove from heat. Add in the 1/4 teaspoon baking soda and 1/4 teaspoon fine sea salt. Pour over the cereal on the sheet pan and, working quickly, stir until cereal and nuts are coated in the mixture.
CINNAMONSUGAR: In a small bowl, combine 2 teaspoons cinnamon and 2/3 cup white sugar and stir. Sprinkle over cereal mixture.
BAKE: Bake for 5 minutes, remove from the oven, and toss using a metal spatula. Return to the oven and bake for 3 more minutes. Remove from the oven and let cool.
CHERRIES AND PRETZELS: Once the mixture has cooled, sprinkle the tart cherries over the cereal. Break up the yogurt pretzels and add. Toss everything together, breaking up the cereal mixture as needed.