In a heavy saucepan, over low heat, melt peanut butter chips with 1 cup sweetened condensed milk, butter and salt.
In a small saucepan, over low heat, melt chocolate chips with remaining sweetened condensed milk. Remove from heat; add vanilla extract. Stir until smooth.
Spread peanut butter mixture evenly into waxed paper lined 8- or 9-inch square pan. Spoon chocolate mixture over peanut mixture. With a table knife or metal spatula, swirl through top of the fudge. Chill 3 hours or until firm. Turn fudge onto a cutting board; peel off paper and cut into squares. Store leftovers covered in the refrigerator.