If you intend to pass parameters between mashups one of the ways to do it is through the Mashup Parameters. First step is to create the parameters: When you select in the Composer the Mashup itself, I mean the top-most element, not a panel, and click on the small arrow on the top left, you will have an entry called Mashup Parameters. There you can go and define any parameter you want. To set the parameters: When you are in another Mashup, and want to call the original one while setting some values at the same time, you will notice that the Navigate widget is dynamically modifying itself in order to display bindable input parameters of the original Mashup.
A shorter version of my explanation is also provided in section 04.02.02 of the Wiki, it is useful to check there also.