Thanks I will investigate into services soon. Right now I have an event A so the second module can listen to. When the second module is "done" it raises another event so the first module know it's finished.
Example : Module A needs a customer being selected which is taken care by module B. Module A raise event "SelectCustomer". Module B listen, process and then raise event "CustomerIsSelected"
It works with events...but i'm sure services would be more elegant or maybe ActionCatalog which I have yet to look at