You will need to run one report through Monarch first and save it as an Excel file.
Then, open the second report in Monarch, and add an External Lookup to the first Excel file based on the common key.
Now you can add a filter that is based on the value from the second report being equal to the lookup value from the first report.
You don't say which version of Monarch 8 you have, but if you have the Pro version, you can link your report to the excel file using the external lookup feature. Linking to external databases is only available in the Pro versions of Monarch. Once done, you can apply a filter to get the results you need.