What I would do to use two different data sources in one query is to use the VDP Server. I would configure it as a vdp data source on the testing tool. After that, on the VDP Admin tool, I’d create two base views over the desired SQL Server and DB2 data sources. That way I would be able to use both data sources for my tests. I would use the [Denodo Testing Tool User Manual](https://community.denodo.com/docs/html/document/denodoconnects/7.0/Denodo%20Testing%20Tool%20-%20User%20Manual) to handle this.
This being said, the Denodo Testing Tool is conceived as a way to test Data Virtualization results against a set of known results and not as a Data Virtualization tool itself. Joining two tables on different data sources is a job for the VDP and the test should be compared against a pre-known result better than creating that result.
Hope this helps!