All you have to do is make a regular chocolate fondue, then add a bit of white chocolate just before you are ready to serve.
1 cup heavy cream
10 ounces dark chocolate, grated
1 teaspoon vanilla
2 ounces white chocolate, grated
Warm cream over low heat, then slowly mix in chocolate until melted.
Remove from heat and stir in vanilla. Transfer to serving pot.
Sprinkle white chocolate on top, then draw a knife through the mixture a few times, just enough to give it a marbled look.