an immersion blender or a drill with an adjustable chuck plus a beater from an electric hand mixer; 2 taps
For the bottom keg: Choose the wider of the two watermelons. Cut a thin slice from the bottom of the watermelon's rind (but not into the flesh) so that the watermelon will sit upright. Invert an 8-inch-wide bowl over the top, like a hat. Using the bowl as a guide, score around the circumference of the watermelon with a serrated knife. Remove the bowl and use the scored line as your guide to cut the top off the watermelon. Carve a line between the flesh and rind as far down as you can go and score down the center of the flesh. Use a large serving spoon to scoop out the flesh in two big pieces as best as you can. Slice the scooped flesh into 1-by-7-inch sticks and transfer to a small baking sheet. Freeze until firm, about 4 hours. (These are your cocktail stirrers.)
Bevel the edge of the watermelon with a paring knife, holding it at a 45-degree angle. Place the second watermelon on top to check the fit. Trim the bottom watermelon as needed so the second watermelon rests securely on top.
For the top keg: Place the second watermelon in a small bowl to stabilize it, and with a sharp paring knife, carve out a circle in the top to make a lid as you would when carving a jack-o'-lantern. Puree the flesh of this watermelon using an immersion blender or drill technique, explained below.
To prepare with an immersion blender: Set the watermelon in the sink (to avoid splashes). Remove the lid and insert the immersion blender into the flesh. Gently push and move the immersion blender down and around in the watermelon to puree the center. As more of the watermelon becomes pureed, move the blender to the sides to puree the flesh, leaving just a thin wall of flesh lining the rind.
To prepare with a drill: Fasten one beater from an electric hand mixer to the tip of a drill and tighten the chuck. Set the watermelon in the sink (to avoid splashes) and push the beater into the flesh. Gently push and move the beater down and around in the watermelon to puree the center. As more of the watermelon becomes pureed, move the beater to the sides to puree the flesh, leaving a thin wall of flesh lining the rind.
Pour the pureed juice from the watermelon through a fine mesh strainer, discarding any pulp and seeds; you should have 6 to 7 cups of strained juice.
Insert a tap into each of the watermelons according to the package directions. Pour the watermelon juice, grenadine, lime juice and 2 cups tequila in the bottom keg. Pour the orange juice and remaining tequila in the top keg. Chill for 2 hours before serving.
To serve: Stack the kegs. Fill tall glasses with ice. Pour in equal parts of the orange mixture from the top keg, then the watermelon mixture from the bottom keg and a top-off of cold seltzer. Serve each drink with a frozen watermelon cocktail stirrer.
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