Preheat oven to 250 degrees. Place popcorn and macadamia nuts in a large bowl.
Melt butter in a heavy saucepan over medium heat. Stir in the syrup, brown and granulated sugars, and 1/2 teaspoon salt; cook until sugars dissolve, stirring once or twice. Raise heat to high and bring to a boil; do not stir again. Boil without stirring until temperature reads 248 degrees on a candy thermometer, 2 to 4 minutes, to make caramel.
Pour caramel over the popcorn mixture, stirring to coat thoroughly. Transfer popcorn to 2 rimmed baking sheets.
Bake until coating is hardened, 40 to 45 minutes, stirring twice (test by removing a few kernels of popcorn from oven; if it crisps within 1 minute of standing at room temperature, it is ready). Remove from oven and immediately sprinkle with remaining 1 teaspoon salt. Let cool completely before breaking up and transferring to airtight storage containers.
Be very careful when pouring the caramel, as it can cause burns if it splatters.
Store for up to one week.