Formatting multiple subgraphs - pydot

Question

I am new to pydot which I would use to create a graph. I would like to format the nodes so as to have blocks of nodes in the same line. I accomplished that using subgraphs, and settings the rank to be the same.

import pydot

# define a graph
graph = pydot.Dot(graph_type="digraph")

# define nodes
node_rec = pydot.Node(label = "Start",shape = "box")
node_bs = pydot.Node("bs")
node_man = pydot.Node("man")
node_dec = pydot.Node("dec")
node_ms = pydot.Node("ms")
node_gtp = pydot.Node("gtp")
node_des = pydot.Node("End",shape = "box")

# add nodes
graph.add_node(node_rec)
graph.add_node(node_man)
graph.add_node(node_gtp)
graph.add_node(node_des)

# add edges
graph.add_edge(pydot.Edge(node_rec, node_bs))
graph.add_edge(pydot.Edge(node_rec, node_man))
graph.add_edge(pydot.Edge(node_bs, node_dec))
graph.add_edge(pydot.Edge(node_dec, node_ms))
graph.add_edge(pydot.Edge(node_ms, node_gtp))
graph.add_edge(pydot.Edge(node_gtp, node_man))
graph.add_edge(pydot.Edge(node_man, node_des))

# create a subgraph
layer1 = pydot.Subgraph(rank='same')
layer1.add_node(node_bs)
layer1.add_node(node_dec)
layer1.add_node(node_ms)
graph.add_subgraph(layer1)

# create figure
graph.write_png('output_ok.png')

However, if I want to have two subgraphs, one on top of each other, I fail to grasp what would be the proper code. Here an example that uses the same nodes, builds two subgraphs with different ranks, and yet it fails.

# add nodes
graph.add_node(node_rec)
graph.add_node(node_des)

# add edges
graph.add_edge(pydot.Edge(node_rec, node_bs))
graph.add_edge(pydot.Edge(node_rec, node_man))
graph.add_edge(pydot.Edge(node_bs, node_dec))
graph.add_edge(pydot.Edge(node_dec, node_ms))
graph.add_edge(pydot.Edge(node_ms, node_gtp))
graph.add_edge(pydot.Edge(node_gtp, node_man))
graph.add_edge(pydot.Edge(node_man, node_des))

layer1 = pydot.Subgraph(rank='source')
layer1.add_node(node_bs)
layer1.add_node(node_dec)
layer1.add_node(node_ms)

graph.add_subgraph(layer1)

layer2 = pydot.Subgraph(rank='max')
layer2.add_node(node_man)
layer2.add_node(node_gtp)

graph.add_subgraph(layer2)

# create figure
graph.write_png('output_bad2.png')

Any idea where I am getting this wrong?