Have you tried looking in the Forums, scan for "MultiCore" this should give a a good Q'nA overview, also
you will notice under the multicore Framwork section a Release Notes as well as the API Docs.
to give you a starting point I have scaned the forums and sniped a few quotes
you don't need MultiCore if you want your entire loaded module to be treated as a single view component with a single mediator for it.
The reason for MultiCore is so that an entire set of the Core actors can run in the same VM with another one, (possibly written by a different vendor without coordination) without having collisions in Notification namespace. These Cores are isolated and do not communicate via notifications sent through a shared Facade for this reason.
The MultiCore demo creates an interface called IWidget that the W1idgetShell in the main app uses to communicate with any Widget. It also creates an interface called IWidgetShell which the Witget uses to communicate with the WidgetShell. That's it. No need to come up with a scheme to be sure notification names are unique inside the two Cores, they call interface methods.
The MultiCore version is still 'experimental' because I haven't yet had the chance to port the unit tests. It has all the same functionality as the standard version and. Knowing the changes I had to mak I feel very confident. But feeling confident and seeing the green bar in FlexUnit are 2 different things, thus the current status
Hope that helps