Is it possible to modify two same objects at the same time (placed in different zones) as when using the array modifier? I tried to split them using Alt + D, but nothing happens. If I modify one of the two objects the other remains unchanged. Using an array modifier would be a bit complicated for me as the objects are facing each other but in the opposite direction. The objects are windows placed on two walls facing each other.
When you select one of the objects, selecting one object as your root object, then you are going to want to select your secondary object to link.
Then by pressing Ctrl + L, it will allow you link the object's data to the main object. That will let you edit the main mesh you selected as the root, meanwhile the other mesh will be edited as well.
and apply the object data to the mesh and it should allow you to edit both meshes.
Maybe Collection Instances would serve your purpose, as it replicates everything, including modifiers. Select and move everything related to your window inside a collection (with M key). Then add/create an instance of that collection with Shift+A, last in the ShiftA menu is Collection Instance, choose the one you created and now you have a "duplicate" with everything you do in your original collection.
It has it's pros (doesn't add more vertices to the scene even if you duplicate it) and cons, like it can't be edited, you need to edit the original collection, or location is dependant on original collection (could be good idea to have that one separated, and use and move around only the instances), also, when you create it, be aware that where the 3d cursor is located does affect it, you'll see better if you try...or the Object ID not working with the Object Data node...but, well, sometimes for tasks like yours it can do the job just great.
Hope it helps!