In order to "share files" from stream B (the "source" stream) to stream A (the "target" stream), first you would declare that Stream A is a flow target of stream B.   Then if you open stream B in the Pending Changes view, you will see outgoing changes from the component that contains those files in stream B.   Right click on those changes and select the "deliver" operation.

If you have made further changes to those files in stream B, those additional changes will appear in the outgoing changes folder.   Conversely, if you have made changes to those files in stream A, those changes will appear in the incoming changes folder.   If you have modified the same file in parallel in both Stream A and Stream B, you would need to create a repository workspace that flows with both Stream A and Stream B, accept the changes from A and B into the workspace, resolve the conflicts, and then deliver the changes back to whichever stream should contain the combination.