Pour Jello mixture into medium bowls, red in one and blue in the other. Add 1 cup boiling water to each and stir until Jello has dissolved. Stir in 1 cup cold water to each bowl (see notes below). Pour into square or round pans and refrigerate for at least 4 hours. (The shape of the pans doesn’t matter; just make sure the pans are identical to each other.)
Using a table knife, cut the set jello in small cubes.
Use a large spoon to scoop a layer of blue jello cubes into the bottom third of plastic dessert cups. Spoon a layer of whipped topping on top of the blue jello. Spoon a layer of red jello cubes on top of the whipped topping to fill the last third of the cups. Top with a dollop of whipped topping (and sprinkle with decorations, if using.) Store in fridge.
If frozen, let the whipped topping thaw in the fridge while the Jello is setting up. It takes the same amount of time!
When adding the water, you can use a little less than a cup of cold water for slightly firmer Jello blocks.
Adding the whipped topping and sprinkles just before serving ensures the whipped cream won't melt and the sprinkle colors won't bleed before serving to guests.