Some testing processes may require that all test results be in a single file for processing. When working with parallel testing this could mean each container has it's own test results.
Getting all the files to a single file can be accomplished with a new "collection" job. This would be a job that requires the parallel job and collects all the test reports from the parallel containers through workspaces.
Then you can combine the files within that "collection" job and pass it through via workspaces to your last job/step -- or alternatively you can process them within the "collection" job itself as needed.