Change distance between edges in dot - graphviz

Question

Is there a way to control the spacing between edges in dot, similar to the nodesep attribute in the other graphviz layout engines? I would like to keep using dot as the layout engine.
By edges I mean either multi-edges or multi-coloured edges, like in the following example. I would like to decrease the space between the a->b edges or increase the space between the c->d edges.
digraph G {
nodesep = "0.15"
a -> b [dir=none color="red"]
a -> b [dir=none color="blue"]
a -> b [dir=none color="green"]
c -> d [dir=none color="green:red:blue"]
}
In dot, the nodesep attribute does not have the desired effect.

Answer 1

Assuming a white background, add white (invisible) lines to change the apparent spacing:
digraph G {
nodesep = "0.15"
c -> d [dir=none color="green:white:white:white:red:white:white:white:blue"]
}
Giving:

Answer 2

There seems to be a bug in that code, only one leg is thicker (penwidth).
So, this solution used ports:
digraph G {
// nodesep = "0.15"
splines=false
edge[penwidth=7]
a:sw -> b:nw [dir=none color="red"]
a -> b [dir=none color="blue"]
a:se -> b:ne [dir=none color="green"]
c -> d [dir=none color="green:white:white:white:red:white:white:white:blue"]
}
Giving:

Answer 3

The best solution I've found is to add transparent edges. That way, this solution works regardless of background colour. Since the edges are transparent, overlap is not a problem and all the coloured edges have the same width.
digraph G {
c -> d [
dir=none
penwidth=5
color="red:transparent:transparent:green:transparent:transparent:blue"
]
}
Produces:
This works for an arbitrary number of edges, and the spacing can be controlled by changing the number of transparent edges between each coloured edge.