Binding events might get tricky. On a general note, communicating between two Mashups within the same Master (or a Mashup) can be done by using Mashup Parameters. You would have to create Mashup Parameters on both mashup_A and mashup_b and bind the relevant value to said parameter in each Mashup.
Here's a simple example:
Add a Grid to mashup_A and bind the 'All Data' of GetImplementingThings service based on GenericThing to this Grid. Create a Mashup Parameter called 'name' and bind the 'name' property of 'Selected Rows' of GetImplementingThings to it. Save mashup_A. What we have done now is, we have a grid populated with the Things associated with the GenericThing Template. Each time you select a row, the value in the 'name' column gets stored to the 'name' property we created as a Mashup Parameter.
In mashup_B, add a Grid and bind 'All Data' of a Dynamic GetPropertyValues service based on any Thing to it. Similar to mashup_A, create a Mashup Parameter called 'name' and bind this property to 'EntityName' of your GetPropertyValues service. In order to trigger this, add a button with the clicked event bound to the same GetPropertyValues service.
Now, go to your Master mashup, and from the tiny dropdown menu of your Contained Mashup (mashup_A) drag and drop the 'name' property on to the other Contained Mashup that has mashup_B assigned to it. When you view this Mashup, and you select a row on mashup_A, clicking on the button on mashup_B should populate the grid on mashup_B with that Thing's properties.
Okay, explaining this was not as simple as I thought