Add drained cherries and 2 Tablespoons of their juices to a small saucepan. In a small dish, mix 2 Tablespoons of cornstarch and 1/3 cup sugar, add to cherries and toss to combine. Heat over medium-high heat, stirring gently, until the sugar and cornstarch are dissolved, and the mixture begins to bubble and thicken. Transfer to a bowl to cool.
Clean the pan and add the drained blueberries and 1/4 cup of the reserved syrup. Toss with the remaining 2 Tablespoons of cornstarch and cook the same way as the cherries. Transfer to a bowl to cool.
Whip the softened cream cheese with the powdered sugar, until very soft and fluffy. Whip the cream in a cold bowl until soft peaks form, then add the whipped cream cheese and continue whipping until fully combined & the cream holds stiff peaks.
Assemble the cake. Butter an 8"-9" square pan and spread a thin layer of the whipped cream on the bottom of the pan. Next, arrange a layer of graham crackers across the cream in a single layer, breaking them to fit. Spread the blueberries over the graham crackers in an even layer, then top with a thick layer of the whipped cream mixture. Top the whipped cream with a second layer of graham crackers, then spread the cherries onto the crackers, followed by another thick layer of the whipped cream. Smooth the cream all the way to the edges with an offset spatula, then crush the remaining crackers into fine crumbs and sprinkle over top.
Chill uncovered in the refrigerator for at least 6 hours, or overnight. To serve, dip a sharp knife in warm water and cut carefully into squares. Clean the knife with the warm water between cuts to preserve the smooth lines. Enjoy!