Allow sherbet to soften at room temperature until it's soft enough to stir, approximately 15 minutes. Transfer to a large mixing bowl and stir in the mango and pineapple. Cover and return to the freezer until ready to serve.
Fifteen minutes prior to serving, remove the sherbet mixture from the freezer to soften. Scoop 4 small balls into a glass, filling approximately half of the glass. Then add 1 teaspoon of the lime juice and fill the remaining portion of the glass with the chilled seltzer water. Garnish with 1/4 teaspoon lime zest. Repeat with the remaining sherbet, lime juice, seltzer and zest.