How can I create OTBI report using union

Summary:

I am trying to create report using 2 subject areas using union all operator. I want to add columns from both the subject areas in result tab but not getting an option to add columns from both the subject areas.

Anyone knows how to do it please let me know.

Content (please ensure you mask any confidential information):

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):