I would say it is not supported at this time. MWS was simply not designed to do this in this way and has implemented its own workspace tabs concepts in the main window.
Basically, the server-side state for the managed beans in the portlet instances are currently stored in a “pageflow” container that is per-session. Which means that requests from other browser tabs in the same session can easily stomp on the server-side state if the portlet instances on the page overlap with what is in other browser tabs.
For future releases, the JSF 2.x specification provides some solutions for this family of problems via new managed bean scopes (ViewScope / ConversationScope / FlashScope) combined with new support for a client window identifier, but due to time constraints and other higher priorities no work has been done yet to integrate these new concepts into CAF + TaskUI. Last time I checked, this didn’t work correctly in CAF portlets and would require some enhancements to the CAF framework to support it properly.
Feel free to file a feature request to let the managers know that this is a feature that you would like to see implemented @ https://empower.softwareag.com/Products/FeatureRequestsInBrainstorm/default.asp