A controller action returns a View, that view could be a whole page, or part of a page, perhaps inserted into a region.
There's no reason you can't still use actions like this, loading each of your composite page regions.
However I've not actually used it like this; from memory Paul had an example on his blog about configuring Prism with Magellan though.
At the end of the day the view models of the views in different regions need to communicate and that is brokered via an event aggregator or bus, rather than a controller action.
Steven Nagy
Readify | Senior Consultant | MVP Windows Azure
M: +61 404 044 513 | E: steve...@readify.net | B: azure.snagy.name