01 - Combine the cubed watermelon, lime juice, and sweetener if using in a blender. Purée until completely smooth.
02 - Pour the watermelon purée evenly into popsicle molds, filling each about two-thirds full. Tap the molds gently on the counter to release any trapped air bubbles.
03 - Place the molds in the freezer for about 1 hour until the surface is slightly firm but not fully solid.
04 - While the watermelon layer sets, combine the peeled kiwi slices, lime juice, and sweetener if using in a blender. Purée until smooth.
05 - Once the watermelon layer has firmed up, pour the kiwi purée on top in each mold, filling almost to the rim. Insert popsicle sticks into the center of each mold.
06 - Return the molds to the freezer and freeze for at least 3 hours, or until the pops are completely firm throughout.
07 - Run the outside of each mold under warm water for a few seconds to loosen, then gently pull the pop free. Serve immediately.