Communicate between two separate modules

I am creating a unity package which can be used in multiple projects. So the most important thing in that is decoupling and independently working of the package. I want to create it in a way that we just have to import the package and while main app is running the package module will be inactive/not initialized but when its needed the main app call for it and that module initialized and start doing its job and after complete its job it should inform the main app that its job has been completed and the result is passed to main game. Most of the people are suggesting to implement Event aggregator in other question/forums some what resemble to my issue. But the issue in that after reading Event Aggregator its looks like its a one way communication because only publisher can trigger the event and subscriber will only listen. But in my case i have two way communication, first Main app will call for initialize the module and then that package module pass the data to back to main app after completing job.