Cut the banana into slices. Finely dice the kiwi, then mix with the banana, lemon juice, and the jam sugar. Cover and let stand for 3 hours. Place the fruit mixture into a pot, and coarsely puree with an immersion blender. Bring the mixture to a boil, stirring constantly.
Cook the mixture for 3-4 minutes, then fill the prepared jars to the brim with the hot jam. Seal tightly and turn upside down to cool for 20 minutes. Turn right side up and cool completely.