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. Gently tap the molds on the counter to release any trapped air bubbles.
03 - Place the filled molds in the freezer for about 1 hour, or until the surface is slightly firm to the touch.
04 - While the watermelon layer sets, blend the peeled kiwi slices, lime juice, and sweetener if using until smooth.
05 - Once the watermelon layer has firmed up, pour the kiwi purée on top, filling each mold nearly to the brim. 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 solid throughout.
07 - Run the outside of each mold under warm water for a few seconds, then gently pull the popsicle out by the stick. Serve immediately.