https://sapui5.hana.ondemand.com/#/api/sap.ui.core.EventBus%23overview
In the receiving controller you need to subscribe your eventId and function you want to call from the second controller:
// Attaches an event handler to the event with the given identifier on the given event channel
this.getOwnerComponent().getEventBus().subscribe("Default", "myEventId", () => {
this._myFunctionIWantToCall();
});
The sending controller has to publish the event to trigger the function call:
// Fires an event using the specified settings and notifies all attached event handlers.
this.getOwnerComponent().getEventBus().publish("Default", "myEventId", {});