Darren - 

this is already supported by the line format editor. See the circled areas below as they let you choose the arrowhead style for each end of a line:

Kiron

@Kiron Bondale Many thanks! I have been playing around in the line format editor for days, not realizing that the answer lay in changing the "None" menu to an arrow ending in the opposite direction! Duh! :)

This could use some UI improvement. It is ambiguous what None refers to.

This allows you to eliminate arrows, but it doesn’t allow multi directional arrows, it seems, which is what I’m looking for, to demonstrate how a flow goes both ways.
 

You can do this by layering arrows, but that’s not great. 

Thanks for listening!

Edit: Ah, just found this https://community.miro.com/ask-the-community-45/double-headed-arrows-4417

This is an old topic that has not yet been addressed by Miro. Everybody else in the world supports lines with arrowheads at each end: Word, Excel, PowerPoint, Visio, AutoCAD, etc. It is an expected and needed feature.

@Mitch D. This does actually exist in Miro, I discovered, it’s just that the UI is a bit confusing. The UI allows you to control both ends of the line independently and to reverse the direction.

In this example, there is no arrow defined on the left side — “none” — so there’s no arrowhead on the left side. But if you click on that and select an arrowhead, there will be. Similarly, on the right side, if you deselect the arrowhead and move it to “none” it will adjust to no arrowhead.