Combine sugar and water in medium saucepan. Bring to boil, stirring to dissolve sugar. Remove from heat. Cool sugar syrup completely.
Blend plums and raspberries in processor until smooth. Strain through coarse sieve into large bowl, pressing on solids to extract as much liquid as possible. Stir in cooled sugar syrup. Refrigerate until chilled, about 2 hours.
Process sorbet in ice cream maker according to manufacturer’s instructions. Transfer sorbet to bowl. Cover and freeze. (Sorbet can be made 1 week ahead. Keep frozen.)