Combine 2 reports with common filter into 1

Summary

Combine 2 reports with common filter into 1

Content

I have 2 custom object Primary & Additional employment. I have created 2 matrix reports to display data for Primary & Additional employment with ballotID as filter. There is no relationship between these 2 objects.

How to combine 2 reports into 1?  I am looking for something like union in SQL Server.