A two-direction arrow makes no sense, or is to be interpreted as a directionless channel
A directed link can always be interpreted as an undirected link by ignoring the arrowhead. In many network displays we have the arrowhead turned off
An Arrowhead Represents a Purpose
Nodes, links, purpose, and time is our general model for networks and virtual teams proved in practice over 25 years of development and application. It now appears that purpose is an aspect of directed links. Directed links are purposeful links. Directed networks are purposeful.
A directed link has a body and direction
An arrowhead gives a  link dynamism, representing a verb of action, rather than just a passive link channel
You can’t tell direction and meaning without knowing the link body and the source point
What does a line mean, with or without arrowheads?
a passive channel (media), a potential connection, an undirected link
a two-headed animal, or dynamic stand-off
bi-directional channels, interaction, two directed links