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

chrome/browser/chromeos/login/screen_locker.cc

Index: chrome/browser/chromeos/login/screen_locker.cc
diff --git a/chrome/browser/chromeos/login/screen_locker.cc b/chrome/browser/chromeos/login/screen_locker.cc
index 3b285a453d472f419d3424cd7316744e96b6bee2..ce66a73965729f532a093ada1960c91963ef6301 100644
--- a/chrome/browser/chromeos/login/screen_locker.cc
+++ b/chrome/browser/chromeos/login/screen_locker.cc
@@ -7,6 +7,8 @@
 #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"
@@ -143,6 +145,9 @@ ScreenLocker::ScreenLocker(const User& user)
 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_);
 }
 
@@ -318,6 +323,9 @@ void ScreenLocker::InitClass() {
 ScreenLocker::~ScreenLocker() {
   DCHECK(MessageLoop::current()->type() == MessageLoop::TYPE_UI);
   ClearErrors();
+  ash::Shell::GetInstance()->
+      desktop_background_controller()->
+          MoveDesktopToUnlockedContainer();
 
   screen_locker_ = NULL;
   bool state = false;