Add enough water to the reserved juice to make 2 cups total and bring to a boil in a small saucepan. Place in a medium bowl:
1 3-ounce package orange-flavored gelatin
1 3-ounce package pineapple-flavored gelatin
Add the hot liquid and stir until the gelatin is dissolved. Stir in:
2 cups cold water
Refrigerate until the gelatin is as thick as raw egg whites, 1 to 1 1/2 hours. Fold in the reserved pineapple along with:
4 cups grated carrots
Rinse an 8-cup mold or bowl, then shake out the excess water. Pour in the gelatin mixture, cover, and refrigerate until set, about 3 hours. Unmold or serve from the bowl.