When you set-up the SharePoint 2013 Workflow Manager, one of the last things you do is run the Register-SPWorkflowService SharePoint PowerShell command to hook it up to a Web Application.
Which is usually something like:Register-SPWorkflowService -SPSite <URL> -WorkflowHostUri 'https://wfhost:12990' -AllowOAuthHttp
But what if you want to hook it up to a 2nd web app?
Well, run basically the same command, but add a ScopeName to the 2nd entry and also use the Force parameter.
Register-SPWorkflowService -SPSite <URL #2> -WorkflowHostUri 'https://wfhost:12990' -AllowOAuthHttp -ScopeName <some text here> -Force
You should be able to verify that it took by going to 'https://wfhost:12990/<scopename text used above>