1. Line a 13 x 9 x 2-inch pan with nonstick foil. Combine white chocolate chips and 1 can of the condensed milk in a medium saucepan. Combine semisweet chips and remaining condensed milk in a different saucepan. Add 1/4 tsp baking soda to each pan. Heat both over medium-low heat until chocolates are melted and smooth, about 5 to 7 minutes, stirring frequently.
2. Remove pans from heat and stir 1/2 cup chopped walnuts into each. Add vanilla to semisweet chocolate mixture. Spoon alternate mounds of chocolate mixtures into prepared pan. Swirl together with a knife. Refrigerate for 3 hours or until firm, then lift from pan with foil and cut into squares.