In a large saucepan, combine the rhubarb, 3/4 cup water, 6 tablespoons sugar and lemon juice. Bring to a boil. Reduce heat; cover and simmer for 10-12 minutes or until rhubarb is tender. Strain; return to the pan and set aside.
In a small bowl, sprinkle gelatin over 1/4 cup water; set aside. Place the raspberries and remaining water in a blender or food processor; cover and process until pureed. Strain; add to rhubarb mixture. Stir in softened gelatin. Cook over low heat for 3-5 minutes or until gelatin is dissolved, stirring occasionally.
In a small bowl, beat cream and remaining sugar until stiff peaks form. Fold into rhubarb mixture. Spoon into dessert glasses or bowls. Cover and refrigerate for 4 hours or overnight.