I contacted Spinning Leaf to ask how I could avoid the white choc weeping/melting a bit into the dark choc. Here is the reply:
We use two different material for casting two-tone chocolates. We use a white candy melt material, and a genuine chocolate for the base. The candy melts have a higher melting point than the chocolate, so it is more stable when casting the cooler melted chocolate over it. You do still want to be sure that your chocolate is as cool as possible when poured over the white. Even though it melts at a cooler temp, it can still be overheated and re-melt the white. Also, avoid tapping the mold to much or too hard to settle the chocolate and remove bubbles, this can jar the cast and also cause the colors to bleed. A few light taps is all that we recommend.
We use two different material for casting two-tone chocolates. We use a white candy melt material, and a genuine chocolate for the base. The candy melts have a higher melting point than the chocolate, so it is more stable when casting the cooler melted chocolate over it. You do still want to be sure that your chocolate is as cool as possible when poured over the white. Even though it melts at a cooler temp, it can still be overheated and re-melt the white. Also, avoid tapping the mold to much or too hard to settle the chocolate and remove bubbles, this can jar the cast and also cause the colors to bleed. A few light taps is all that we recommend.