Stir the sugar and water in a large saucepan over medium heat until the sugar dissolves, about 5 minutes. Cool completely.
Puree the peaches and the orange peel in a blender with 1/2 cup of the sugar syrup until smooth. Strain through a fine-meshed strainer and into a bowl. Cover and refrigerate. In a clean blender, puree the blueberries with 1/3 cup of the sugar syrup until smooth. Strain through a clean fine-meshed strainer and into a bowl. Discard the solids and the seeds.
For each serving, pour 2 to 4 tablespoons of the desired fruit puree into a champagne flute. Slowly pour enough Prosecco into the flute to fill. Gently stir to blend.
Do-ahead Tip: The fruit purees can be made 1 day ahead. Cover separately and refrigerate.