Custom editor field accepting multiple drag and drops?

I have a custom editor I'm using for a custom object. Right now I can drag an object into that field and immediately add it to a list as so:

 customObject = EditorGUILayout.ObjectField("Drag Element Here to Add", 
     customObject, typeof(Element), false) as Element;
             
     if (customObject != null) {
         if (!objectList.Contains(customObject)) {
             objectList.Add(customObject);
             customObject = null;
         } else {
             Debug.Log(customObject.name + " is already in the list.");
             customObject = null;
         }
     }

I can't seem to drag and drop more than one thing in, no matter what I've tried (like using a list customObject instead of just a single object), but I've seen it done (for instance with 2dTK). How?