Thanks for your post. If you have the line selected you can modify both ends in the properties bar above the editor. Please see our Line Style tutorial below for more detail. https://help.lucid.co/hc/en-us/articles/16157138194836-Add-and-style-lines-in-Lucidchart

Thom as you've probably noticed this thread is a bit old. To make a line double-arrowheaded simply select the line. Then on the top properties bar there are two drop-down menus for changing the line endings. One reads 'none' if the line is single-arrowheaded. Check out the highlighted drop-down menus in the screenshot below. Make sure both of them have the arrowhead ending that you desire and you'll have a double-arrowheaded line! 

Ah!  Thanks Monika.  I'm surprised I didn't stumble across it myself.  I guess I've spent more time in Visio and Powerpoint and less in Illustrator or other tools that use separate controls.  But looking at other UIs with separate controls I notice that it's more obvious with others than with Lucid.  With Google Docs Drawing for example the icons for "none" still make it clear that the control is related to line ends.  And with Illustrator there is a label and placement that make it clear.  

Perhaps adding a pipe character  at either end of the arrow-related controls to indicate that they are a group or using an icon like the one that google draw uses instead of the word "none" would help newbies like me make the association faster.  

Cheers