Preheat the oven to 350°F (175°C).

Grease your loaf pans with cooking spray or butter.

In a large mixing bowl, whisk together the granulated sugar, brown sugar, and vegetable oil until smooth. Add the eggs and milk, mixing until fully combined.

In a separate bowl, whisk together the flour, baking powder, salt, and ground cinnamon.

Gradually add the dry mixture to the wet mixture, stirring gently with a rubber spatula.

In a small bowl, mix together the remaining brown sugar and ground cinnamon.

Pour half of the batter into each prepared loaf pan. Sprinkle half of the cinnamon mixture on top, followed by the remaining batter and the rest of the cinnamon mixture.

Using a knife, gently swirl through the batter to create a marbled effect.

Bake in the preheated oven for 50 to 60 minutes, or until a toothpick inserted comes out clean.

Let the loaves cool in the pans for about 10 minutes, then transfer to a cooling rack to cool completely.