Force different ranks for subgraphs in Dot (graphviz)

Question

I have a dot graph with two subgraphs with the option "rank=same"

digraph G {
rankdir=LR;
newrank=true;
splines=false;

subgraph  {
rank=same;
a1;
b1;
c1;
}

subgraph  {
rank=same;
a2;
b2;
c2;
}
}

It gives me the following :

However, how could I force the ranks to be different, i.e. for a1, b1, c1 to be in a different rank than a2, b2, c2 ?

Answer 1

Use an invisible edge:

digraph G {
rankdir=LR;
//newrank=true;  // only applies to clusters
splines=false;    
subgraph  {
rank=same;
a1;
b1;
c1;
}
subgraph  {
rank=same;
a2;
b2;
c2;
}
a1 -> a2 [style=invis]
}

Giving: