To make the lemon syrup, bring one-fourth cup sugar, lemon juice and one-fourth cup water to a boil in a small saucepan over medium heat, stirring to dissolve sugar. Let it cool completely then brush it over both sides of cake slices. Cut each slice in quarters. With an electric mixer, beat cream cheese with remaining sugar on high speed until lightened. With mixer on medium speed, gradually add heavy cream in a steady stream; continue beating until mixture is light and airy (mixture will be like a very soft whipped cream).
Arrange half the cake pieces at the bottom of a 2-quart serving dish. Spoon half the cream-cheese mixture over cake in dollops; spread to the sides as well. Scatter half the blueberries and raspberries on top. Repeat layering with remaining cake, cream-cheese mixture and berries, piling berries in the centre. Cover and refrigerate until chilled before serving.