The Designer help documentation briefly describes the procedure in the “User Interface Controls Concepts / Control ID Reference” section like this:
[quote]
External Portlet References
Some controls can accept a reference to another control outside of the current view, in an external portlet on the same portal page. An external portlet reference begins with the URI of, or an alias to, the portlet, followed by a pound sign (#), followed by the absolute control ID reference (sans leading colon). For example, say the “myPanel
#MWS-CAF-Task-Engine#webMethods-BPMS#webMethods