If you use draw.io in Confluence and Jira Cloud, you only need to license it in Confluence Cloud for it to work in both. It does have to be installed on both.
First, go to the Jira Cloud find new plugins page https://xyz.atlassian.net/plugins/servlet/upm/marketplace/highest-rated ( replace xyz with your sub-domain in all URLs on this page ) and install the draw.io for Jira Cloud plugin. If a trial of this plugin automatically starts, stop the trial by locating the plugin in https://xyz.atlassian.net/plugins/servlet/upm?source=side_nav_manage_addons and clicking "stop trial".
Next, go to the Confluence Cloud find new plugins page https://xyz.atlassian.net/wiki/plugins/servlet/upm/marketplace/highest-rated and install the draw.io for Confluence Cloud plugin. If a trial does not automatically start, go to https://xyz.atlassian.net/wiki/plugins/servlet/upm?source=side_nav_manage_addons, expand the draw.io plugin and click "start trial".
Following this process will allow draw.io to work in both Jira and Confluence Cloud, but only bill you for the Confluence Cloud version.