I'm speculating some here, but it may be that one or both of the chocolates were overheated. When you overheat milk chocolate or dark chocolate, the common problems is that the solids clump and it gets grainy. White chocolate is cocoa butter, sugar, and milk solids, including fats. It's a complex mix of fats, all with different characteristics. It's very complicated, and I don't know enough to predict its behavior at particular temperatures. But I'm thinking in terms of oils separating. But I think the next idea is more likely.
It might also be possible that what you have is the result of the chocolate being tempered. That's normally a good thing, especially when molding, because you want it to shrink and release when it cools. It may be that, since you're using two chocolates and letting one layer set before pouring the next, that the first layer has shrunk to it's permanent solid state, and then the next layer is poured on and shrinks as it becomes cool, peeling itself off the first layer. You certainly did get a temper, because it became noticeably hard. The shine and crunch we like in chocolate is achieved by tempering.
Now, here's what might have happened. You will note that bark recipes call for each layer to be cooled in the refrigerator while you heat the next layer. When chocolate is held at about 88-degrees F for a time, it become tempered and behaves well for molding and for appearance and bite. If a first layer is just left out to cool just enough to start to set, you could end up with the second layer, coming out of the pot at about 110-degrees, cooling to around 88 for a time and tempering, therefore shrinking. Perhaps pouring the second layer onto a very cool first layer drops the temperature rapidly past the temper point and allows it to stick.
I honestly don't work with chocolate that much, so a lot of my notions are theory from reading about working with it, so I don't have a good feel for how likely any of this is to explain your problem, because I haven't played enough with chocolate on chocolate layering. But I do know that most chocolate problems come back to temperature control.
Two other things that people have recommended for the separation. One is to wipe down the first layer with a paper towel to remove any separated oil. The other is to let the bark come to room temperature before cutting. The whole discussion above might be so much baloney if it turns out you just need to let it warm up before cutting.
