See attached Image?
If you have two links between a pair of nodes, that is the default routing that you will get. The separation is intentional to make it clearer to the user that there are two links there.
I suggest that you only use a single link between the two nodes. Links do not have to be treated as being directional. Usually one does not even bother having two arrowheads, but if you really want to, that’s OK.
If you really want to use two separate links, you can set or bind the Route.Curviness to zero, so that the link paths are superimposed.