Combine the cherries, cranberries, and coconut sugar in a heavy-bottomed saucepan over medium heat. If using frozen fruit, cook until the fruit has softened. Once the fruit has softened, blend it with an immersion blender to create a pulp.
Increase the heat to medium-high and cook until the fruit juice is bubbly and slightly thick. Turn off the heat and allow the mixture to cool for a few minutes.
Use a fine mesh sieve to strain the mixture and separate the juice from the pulp. Place the juice in the refrigerator to cool, and discard the pulp.
Add a generous handful of crushed ice to two glasses. Divide the juice between the two glasses and top with lime-flavored sparkling water (approximately six ounces each). Add cranberries as garnish, if desired, and serve.