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

chrome/browser/chromeos/login/screen_locker.cc

 // 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.
 
 #include "chrome/browser/chromeos/login/screen_locker.h"
 
 #include <string>
 #include <vector>
 
+#include "ash/desktop_background/desktop_background_controller.h"
+#include "ash/shell.h"
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/lazy_instance.h"
 #include "base/memory/weak_ptr.h"
 #include "base/message_loop.h"
 #include "base/metrics/histogram.h"
 #include "base/string_util.h"
 #include "base/timer.h"
 #include "base/utf_string_conversions.h"
 #include "chrome/browser/chromeos/login/authenticator.h"
@@ skipping 116 matching lines @@
       start_time_(base::Time::Now()),
       login_status_consumer_(NULL),
       incorrect_passwords_count_(0) {
   DCHECK(!screen_locker_);
   screen_locker_ = this;
 }
 
 void ScreenLocker::Init() {
   authenticator_ = LoginUtils::Get()->CreateAuthenticator(this);
   delegate_.reset(new WebUIScreenLocker(this));
+  ash::Shell::GetInstance()->
+      desktop_background_controller()->MoveDesktopToLockedContainer();
   delegate_->LockScreen(unlock_on_input_);
 }
 
 void ScreenLocker::OnLoginFailure(const LoginFailure& error) {
   DVLOG(1) << "OnLoginFailure";
   content::RecordAction(UserMetricsAction("ScreenLocker_OnLoginFailure"));
   if (authentication_start_time_.is_null()) {
     LOG(ERROR) << "authentication_start_time_ is not set";
   } else {
     base::TimeDelta delta = base::Time::Now() - authentication_start_time_;
@@ skipping 155 matching lines @@
 void ScreenLocker::InitClass() {
   g_screen_lock_observer.Get();
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 // ScreenLocker, private:
 
 ScreenLocker::~ScreenLocker() {
   DCHECK(MessageLoop::current()->type() == MessageLoop::TYPE_UI);
   ClearErrors();
+  ash::Shell::GetInstance()->
+      desktop_background_controller()->MoveDesktopToUnlockedContainer();
 
   screen_locker_ = NULL;
   bool state = false;
   content::NotificationService::current()->Notify(
       chrome::NOTIFICATION_SCREEN_LOCK_STATE_CHANGED,
       content::Source<ScreenLocker>(this),
       content::Details<bool>(&state));
   DBusThreadManager::Get()->GetPowerManagerClient()->
       NotifyScreenUnlockCompleted();
 }
@@ skipping 12 matching lines @@
   bool state = true;
   content::NotificationService::current()->Notify(
       chrome::NOTIFICATION_SCREEN_LOCK_STATE_CHANGED,
       content::Source<ScreenLocker>(this),
       content::Details<bool>(&state));
   DBusThreadManager::Get()->GetPowerManagerClient()->
       NotifyScreenLockCompleted();
 }
 
 }  // namespace chromeos