Accepting drag and drop to multiple layout controlled areas in Unity Editor scripts.

MultiDragAndDropExample.cs

using UnityEngine;
using UnityEditor;
using System.Collections;
using System.Text;

public class MultiDragAndDropExample : EditorWindow 
{
    [MenuItem("Examples/Multi Drag and Drop")]
    public static void ShowWindow()
    {
        MultiDragAndDropExample example = ScriptableObject.CreateInstance<MultiDragAndDropExample>();
        if (null != example) example.Show();
    }

    /// <summary>
    /// Handle Unity's OnGUI call
    /// </summary>
    void OnGUI()
    {
        // Draw first area
        BoxWithEventsOnGUI("First");

        // Draw second area
        BoxWithEventsOnGUI("Second");
    }

    /// <summary>
    /// Draws a vertical layout box and handles it's events
    /// </summary>
    /// <param name="label">Label for the box</param>
    void BoxWithEventsOnGUI(string label)
    {
        // Draw the controls
        EditorGUILayout.BeginHorizontal("Box");
        {
            GUILayout.Label(label);
        }
        EditorGUILayout.EndHorizontal();

        Rect lastRect = GUILayoutUtility.GetLastRect();

        // Handle events
        Event evt = Event.current;
        switch (evt.type)
        {
            case EventType.DragUpdated:
                // Test against rect from last repaint
                if (lastRect.Contains(evt.mousePosition))
                {
                    // Change cursor and consume the event
                    DragAndDrop.visualMode = DragAndDropVisualMode.Copy;
                    evt.Use();
                }
                break;
            case EventType.DragPerform:
                // Test against rect from last repaint
                if (lastRect.Contains(evt.mousePosition))
                {
                    // Handle the drop however you want to
                    StringBuilder builder = new StringBuilder();
                    builder.Append(label).Append(" accepted items: \n");
                    foreach (string path in DragAndDrop.paths) builder.Append(path).Append("\n");
                    EditorUtility.DisplayDialog("Drag and Drop", builder.ToString(), "Ok");

                    // Change cursor and consume the event and drag
                    DragAndDrop.visualMode = DragAndDropVisualMode.Copy;
                    DragAndDrop.AcceptDrag();
                    evt.Use();
                }
                break;
        }
    }
}