John Willson I believe the free online draw.io (connected to google drive) lets you do two-way relationships, though it’s not designed to label each direction independently. I think we resorted to manual lung inserting the labels.

Omnigraffle isn’t designed specifically with assymetrical relationships in mind (like Visio, it’s either uni- or bidirectional), but it does make having multiple independent connections between the same two objects really easy. So when I did a relationship map with it, I’d just draw the connection ok one direction and label it, and then draw the connection in the other direction and label it. With a couple settings you can have multiple connections automatically avoid each other and be readable. Or you can manually clean them up later. draw.io – Flowchart Maker & Online Diagram Software