Quick Chocolate Pecan Fudge

6 ounces semisweet chocolate, finely chopped
2 ounces unsweetened chocolate, finely chopped
2 tablespoons unsalted butter, cut into 1/2-inch cubes
1 1/2 cups granulated sugar
1/2 cup plus 2 tablespoons heavy cream
3/4 cup Marshmallow Cr