Multiple HDCs into one HDC to Bitblt in MFC

Question

void Draw(int _iPane, HDC hDC, const CRect& scrRect, BOOL bRecalc)
{
    const std::vector<Info>& drawinfos = App::General::Infos();
    std::vector<std::future<void>> futures;

    for (const auto& info : drawinfos)
    {
        futures.emplace_back(std::async(std::launch::async, [=]()
        {
            if (auto dcPtr = ThreadSafeWinGdi::CreateCompatibleDC(hDC))
            {
                HDC threadHdc = dcPtr.get();
                CDC* threadCDC = ThreadSafeWinGdi::FromHandle(threadHdc);
                auto view = static_cast<View*>(App::GetView());

                CRect rect = view->GetViewRect();
                HBITMAP newBitmap = ThreadSafeWinGdi::CreateCompatibleBitmap(hDC, rect.Width(), rect.Height());
                auto oldBitmap = (HBITMAP)(threadCDC->SelectObject(newBitmap));
                auto brush = ThreadSafeWinGdi::CreateSolidBrush(view->GetBkColor(_Env().GetEllipsoid()));
                ThreadSafeWinGdi::FillRect(threadHdc, rect, brush.get());

                CManager manager;
                manager.SetEventWnd(coconutView);
                manager.SetDC(_iPane, threadHdc);
                manager.SetViewRect(_iPane, scrRect);
                manager.SetMapRect(_iPane, App::GetERManager().GetDrawMapRect(_iPane), FALSE);

// ::Polygon, ::Polyline...(thread safe ok)
                DrawInfo(manager, _iPane, info);

                ThreadSafeWinGdi::BitBlt(hDC, 0, 0, rect.Width(), rect.Height(), threadHdc, 0, 0, SRCCOPY);
                ThreadSafeWinGdi::DeleteObject(newBitmap);
            }
        }));
    }

    for (auto& future : futures)
    {
        future.wait();
    }

I would like to process drawing through multithreading. After creating and drawing a dc for each thread, I want to draw it by Bitblt to mainDc. However, it seems that only the bitblt target appears at the end.

ex) ThreadSafeWinGdi code

BOOL ThreadSafeWinGdi::DeleteObject(HGDIOBJ ho)
{
    static std::mutex deleteObject;
    std::unique_lock<std::mutex> lock(deleteObject);

    return ::DeleteObject(ho);
}