2nd display should show the same background as login/lock screen:

ash/shell_window_ids.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ASH_SHELL_WINDOW_IDS_H_
 #define ASH_SHELL_WINDOW_IDS_H_
 
 // Declarations of ids of special shell windows.
 
 namespace ash {
@@ skipping 39 matching lines @@
 const int kShellWindowId_AppListContainer = 9;
 
 // The container for user-specific modal windows.
 const int kShellWindowId_SystemModalContainer = 10;
 
 // The container for input method components such like candidate windows.  They
 // are almost panels but have no activations/focus, and they should appear over
 // the AppList and SystemModal dialogs.
 const int kShellWindowId_InputMethodContainer = 11;
 
+// The container for the lock screen background.
+const int kShellWindowId_LockScreenBackgroundContainer = 12;
+
 // The container for the lock screen.
-const int kShellWindowId_LockScreenContainer = 12;
+const int kShellWindowId_LockScreenContainer = 13;
 
 // The container for the lock screen modal windows.
-const int kShellWindowId_LockSystemModalContainer = 13;
+const int kShellWindowId_LockSystemModalContainer = 14;
 
 // The container for the status area.
-const int kShellWindowId_StatusContainer = 14;
+const int kShellWindowId_StatusContainer = 15;
 
 // The container for menus.
-const int kShellWindowId_MenuContainer = 15;
+const int kShellWindowId_MenuContainer = 16;
 
 // The container for drag/drop images and tooltips.
-const int kShellWindowId_DragImageAndTooltipContainer = 16;
+const int kShellWindowId_DragImageAndTooltipContainer = 17;
 
 // The container for bubbles briefly overlaid onscreen to show settings changes
 // (volume, brightness, etc.).
-const int kShellWindowId_SettingBubbleContainer = 17;
+const int kShellWindowId_SettingBubbleContainer = 18;
 
 // The container for special components overlaid onscreen, such as the
 // region selector for partial screenshots.
-const int kShellWindowId_OverlayContainer = 18;
+const int kShellWindowId_OverlayContainer = 19;
 
 }  // namespace internal
 
 }  // namespace ash
 
 
 #endif  // ASH_SHELL_WINDOW_IDS_H_