prepare both boxes of jello according to jiggler recipe. Pour the mixture into an 8-inch square pan and chill it. Halfway through the chilling time (about 45 to 50 minutes), whisk the gelatin vigorously to create ripples and bubbles, then let it finish chilling.
Stack the cakes, spreading a thin coating of icing between them. Trim the corners to create more of a rounded tub shape. Next, carve out a 1 1/2-inch-deep basin, using a knife to first outline it and a spoon to scoop out the cake. Leave a 2-inch-wide border around the sides and end of the tub and a slightly wider border where the faucets will go.
Frost the tub, including the walls but not the floor of the basin. Now, for a faucet, turn the licorice pipe upside down and insert the thin portion of the stem into the wall of the basin, as shown
For each faucet knob, use kitchen scissors to snip 4 V-shaped notches in the top of a regular-size marshmallow, then gently press a mini marshmallow into each notch, as shown. The freshly cut marshmallow surface will be sticky enough to keep the pieces in place. Place the knobs atop the cake and use decorators' gel to label them H and C and outline the bases.
Spoon the Jiggler water into the basin and set the duck or ducks afloat