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>

 

http://technet.microsoft.com/en-us/library/jj663115%28v=office.15%29.aspx

http://technet.microsoft.com/en-us/library/jj658588%28v=office.15%29