LRESULT CALLBACK WindowProcedure (HWND hwnd, UINT, WPARAM, LPARAM);
LRESULT CALLBACK BoxProcedure (HWND hwnd, UINT, WPARAM, LPARAM);

int WINAPI
WinMain (HINSTANCE hThisInstance,
         HINSTANCE hPrevInstance,
         LPSTR lpszArgument,
         int nFunsterStil)

{
    MSG messages={0};            
    WNDCLASSEX wincl={0};        

    wincl.hInstance = hThisInstance;
    wincl.lpszClassName = szClassName;
    wincl.lpfnWndProc = WindowProcedure;      
    wincl.style = CS_DBLCLKS;                 
    wincl.cbSize = sizeof (WNDCLASSEX);

    wincl.hIcon = LoadIcon (NULL, IDI_APPLICATION);
    wincl.hIconSm = LoadIcon (NULL, IDI_APPLICATION);
    wincl.hCursor = LoadCursor (NULL, IDC_ARROW);
    wincl.lpszMenuName = NULL;                 
    wincl.cbClsExtra = 0;                      
    wincl.cbWndExtra = 0;                      
    wincl.hbrBackground = (HBRUSH) COLOR_BACKGROUND;

     if (!RegisterClassEx (&wincl))
        return 0;

    HWND hwnd = CreateWindowEx (
           0,                   
           szClassName,         
           "Controlwindow",       
           WS_OVERLAPPEDWINDOW, 
           CW_USEDEFAULT,       
           CW_USEDEFAULT,       
           500,                 
           200,                 
           HWND_DESKTOP,        
           NULL,                
           hThisInstance,       
           NULL                 
           );

    WNDCLASSEX box={0};        
    box.hInstance = hThisInstance;
    box.lpszClassName = szBoxName;
    box.lpfnWndProc =BoxProcedure;         
    box.style = CS_DBLCLKS;                   
    box.cbSize = sizeof (WNDCLASSEX);

        box.hIcon = LoadIcon (NULL, IDI_APPLICATION);
    box.hIconSm = LoadIcon (NULL, IDI_APPLICATION);
    box.hCursor = LoadCursor (NULL, IDC_ARROW);
    box.lpszMenuName = NULL;                
    box.cbClsExtra = 0;                      
    box.cbWndExtra = 0;                      

    box.hbrBackground = (HBRUSH) COLOR_BACKGROUND;

    if (!RegisterClassEx (&box))
        return 0;

        HWND picbox = CreateWindowEx (
           0,                  
           szClassName,         
           "Picturewindow",       
           WS_OVERLAPPEDWINDOW, 
           0,       
           0,       
           814,                 
           639,                 
           HWND_DESKTOP,       
           NULL,                
           hThisInstance,       
           NULL                            );

    ShowWindow (hwnd, nFunsterStil);
    ShowWindow (picbox, nFunsterStil);

    while (GetMessage (&messages, NULL, 0, 0))
    {
                TranslateMessage(&messages);
                DispatchMessage(&messages);
    }
    return messages.wParam;
}

großeß thx @flenders