In a double boiler (or on pot set atop another with about an inch of water), bring the water to a simmer in the bottom, and gently melt the white chocolate chips. Note that I do not recommend melting the chocolate chips in the microwave, as it does not tend to melt as smooth.
As soon as the white chocolate is melted, remove from the heat, quickly stir in the pumpkin pie spice and toffee bits, then pour onto the prepared sheet and spread into a think layer with a small offset spatula.
Let harden completely, putting into the refrigerator if necessary.
Melt the semisweet chips in a double boiler or the microwave and transfer to a sandwich bag. Cut off the corner and drizzle over the bark.
Let harden completely, putting into the refrigerator if necessary. Break into pieces and store in an airtight container.