Merge Two Separate Equal Objects to Edit Them at the Same Time 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.
But please be mindful that it likely has to have the same topology.

and apply the object data to the mesh and it should allow you to edit both meshes.