Is it possible to have two parts (same namespace, same class name) to a partial class in separate DLLs?
While other answers do provide the unpleasant "No" that anyone landing on this page didn't want to see or hear, I was struck by another thought that hasn't been mentioned here yet. If partial classes were allowed across assemblies, one would get access to private members of existing types that were not written by him, thus allowing him to manipulate them in ways that were not intended by the original author, thus jeopardizing the functionality of all inheriting classes too.
Not only that, those classes in other assemblies (and their children) would need to be recompiled to make it work. Thus it is logically not possible to allow splitting a class over different assemblies.