In a 4-quart saucepan over medium heat, bring 1/2 cup water, 1 cup sugar, and 1 1/2 pounds fresh (or thawed frozen) rhubarb, sliced into 1-inch-thick pieces, to a simmer. Cook until the rhubarb is tender and starting to fall apart, 6 to 8 minutes. Remove from heat.
Fold in 9 ounces fresh (or thawed frozen) raspberries. Let stand until the mixture thickens, about 10 minutes. Serve, drizzled with heavy cream, if desired.