The simplest solution would be to couple the two apps. When you open app B send the object of app A to it. Then when app B updates x, it can update the property of app A.
A more robust solution would be to uncouple the two apps and add a mediator class. The mediator knows both app A and app B. It passes all the data between the two back and forth. Both apps don't need to know each other, only the mediator.
The reason I say this implementation is more robust is because tomorrow you may need app A to communicate with a third class instead of app B for some specific functionality. In that case you don't need to change app A, only the mediator class.