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

ash/root_window_controller.cc

Index: ash/root_window_controller.cc
diff --git a/ash/root_window_controller.cc b/ash/root_window_controller.cc
index 0ef08bc42a6582810d2f2f09f401a6693785e804..6423965d01acc3591dc1f1b7b30ab1a7af60a21d 100644
--- a/ash/root_window_controller.cc
+++ b/ash/root_window_controller.cc
@@ -173,6 +173,13 @@ void CreateContainersInRootWindow(aura::RootWindow* root_window) {
       non_lock_screen_containers);
   SetUsesScreenCoordinates(input_method_container);
 
+  aura::Window* lock_background_containers = CreateContainer(
+      internal::kShellWindowId_LockScreenBackgroundContainer,
+      "LockScreenBackgroundContainer",
+      lock_screen_containers);
+
+  SetChildWindowVisibilityChangesAnimated(lock_background_containers);
+
   // TODO(beng): Figure out if we can make this use
   // SystemModalContainerEventFilter instead of stops_event_propagation.
   aura::Window* lock_container = CreateContainer(
@@ -277,7 +284,8 @@ void RootWindowController::CreateContainers() {
 
 void RootWindowController::CloseChildWindows() {
   // Close background widget first as it depends on tooltip.
-  root_window_layout_->SetBackgroundWidget(NULL);
+  root_window_layout_->SetBackgroundWidget(NULL,
+      kShellWindowId_DesktopBackgroundContainer);
   workspace_controller_.reset();
   aura::client::SetTooltipClient(root_window_.get(), NULL);