AFAIK you cannot.
Clusters described in the documentation:
If the subgraph name begins with a cluster, Graphviz marks the subgraph as a special cluster subgraph. If supported, the layout engine will make the layout so that the nodes belonging to the cluster are combined , while the entire cluster image contains within the bounding box.
The whole idea of clusters does not overlap, but groups nodes (and graphviz causes an error when node is part of two subgraphs).