How to Accept Incoming Change Sets from 2 flow targets without manually editing the Repository Workspace properties.
2 Components are loaded in the Repository Workspace.
Accepted answer
In Eclipse, in Team -> Jazz_Source_Control -> Changes preferences, in the "Flow Targets" section, select "Show all (advanced).
Comments
If you set the Eclipse preference as specified in this answer, then you do not need to make Stream1 as "Current Flow" for the Repository Workspace ... all incoming flows (current and non-current) will be displayed in the Pending Changes view. So you will see incoming changes in your Pending Changes view if there are incoming changes from any of your flow target streams.
3 other answers
For all I know, this is not supported out of the box. If you need to load multiple streams/workspaces, because you flow ( components ) against multiple streams, for all I know, you would have to create a custom load mechanism e.g. using the SCM Command Line.
The key here is "Build" and I am pretty sure you can not automate multiple flow targets in a build definition using the built in configurartion abilities. Always willing to accept different opinions, with proof though.
Comments
The question is ambiguous. It says "both components contribute to the build", but it does not specify whether he is trying to perform the flows during the build operation, or whether he is performing the flows in a GUI client before requesting the build. Since in the response to my answer, the Pending Changes view is mentioned, this seems to indicate that the flows are being done in the GUI. Also, if he was just automating the builds, it is reasonably straightforward ... he just needs to have the build script perform the flows from various flow targets of the workspace.