Bring 4 cups of water to a boil. In a large bowl, combine 2 cups of boiling water with the package of lime Jello. Stir to dissolve, about 2 minutes. Add 2 cups of chilled melon liqueur and stir to combine. Pour half of the lime jello into plastic cups, reserving the other half. Chill until firm. In another large bowl, combine the remaining boiling water with the package of lemon jello. Stir to dissolve, about 2 minutes. Add 2 cups of the chilled lemon vodka and stir to combine. Carefully pour the lemon jello over the first layer of lime jello. Chill until firm. Layer the remaining lime jello on top of the lemon jello and chill until firm.