Preheat oven to 300 degrees F and adjust oven racks to upper-middle and lower-middle positions. Line two rimmed baking sheet with parchment paper.
In a large bowl, toss together the cereal, pecans and pretzels.
In a medium bowl, whisk together the sugars, cinnamon, salt and cayenne (if using); set aside.
In a small bowl, whisk the egg whites until frothy.
Pour the egg whites over the cereal mixture and toss with a rubber spatula until the mixture is completely coated with egg whites, 1 to 2 minutes. Add the sugar mixture and toss again until everything is evenly coated.
Divide the mixture between the two baking sheets and spread into an even layer. Bake for 45 minutes, stirring each pan and rotating them halfway through baking. Allow to cool completely at room temperature. The snack mix can be stored in an airtight container at room temperature for up to 2 weeks.