01 - Combine the cubed watermelon, 2 tablespoons of lime juice, and sweetener if desired in a blender. Purée until completely smooth.
02 - Pour the watermelon purée evenly into popsicle molds, filling each about two-thirds full. Gently tap the molds on the counter to release any trapped air bubbles.
03 - Place the molds in the freezer for approximately 1 hour, or until the watermelon layer is slightly firm to the touch.
04 - While the watermelon layer sets, blend the peeled kiwi slices, 1 tablespoon of lime juice, and sweetener if using until silky smooth.
05 - Once the watermelon layer has firmed, pour the kiwi purée on top in each mold, filling nearly 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 frozen through.
07 - Run the outside of each mold under warm water for a few seconds, then gently pull the pop free. Serve immediately.