#include <windows.h>
#include "..resource.h"
LRESULT CALLBACK FrameWndProc(HWND hwnd, UINT iMsg,
         WPARAM wParam, LPARAM lParam);
LRESULT CALLBACK ChildWndProc(HWND hDlg, UINT iMsg,
         WPARAM wParam, LPARAM lParam);
HINSTANCE hInst;

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
       LPSTR lpszCmdLine, int nCmdShow)
{
 HWND   hwnd;
 MSG   msg;
 WNDCLASS WndClass;  
 hInst = hInstance;
 WndClass.style = CS_HREDRAW | CS_VREDRAW;
 WndClass.lpfnWndProc = FrameWndProc;  
 WndClass.cbClsExtra = 0;  
 WndClass.cbWndExtra = 0;  
 WndClass.hInstance = hInstance;  
 WndClass.hIcon = LoadIcon(NULL, IDI_APPLICATION);
 WndClass.hCursor = LoadCursor(NULL, IDC_ARROW); 
 WndClass.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH); 
 WndClass.lpszMenuName = MAKEINTRESOURCE(IDR_MENU7_6);  
 WndClass.lpszClassName = "Window Class Name"; 
 RegisterClass(&WndClass); 
 WndClass.lpfnWndProc = ChildWndProc;
 WndClass.lpszMenuName = NULL;
 WndClass.lpszClassName = "Child Window Class Name";
 RegisterClass(&WndClass) ;
 hwnd = CreateWindow("Window Class Name",
  "Main Window Title",
  WS_OVERLAPPEDWINDOW,
  CW_USEDEFAULT, 
  CW_USEDEFAULT, 
  CW_USEDEFAULT, 
  CW_USEDEFAULT, 
  NULL, 
  NULL, 
  hInstance, 
  NULL 
 );
 ShowWindow(hwnd, nCmdShow);
 UpdateWindow(hwnd);
 while(GetMessage(&msg, NULL, 0, 0))
 {
  TranslateMessage(&msg); 
  DispatchMessage(&msg); 
 }  
 return (int)msg.wParam;
}

HWND ChildHwnd[2];
LRESULT CALLBACK FrameWndProc(HWND hwnd, UINT iMsg,
         WPARAM wParam, LPARAM lParam)    
{
  static BOOL split; 
  static HCURSOR hCursor;
  static int boundary = -1;
  RECT rectView;

  switch (iMsg)
  {
 case WM_CREATE:
  split = FALSE;
  return 0;
 case WM_COMMAND:
  switch(LOWORD(wParam))
  {
  case ID_SPLIT_2_1:
   if (split == TRUE) break;
   split = TRUE;   
   GetClientRect(hwnd, &rectView);
   ChildHwnd[0] = CreateWindowEx(
    WS_EX_CLIENTEDGE,
    "Child Window Class Name",
    NULL, 
    WS_CHILD | WS_VISIBLE,
    0, 
    0, 
    rectView.right,
    rectView.bottom/2-1,
    hwnd,
    NULL,
    hInst,
    NULL
   );
   ChildHwnd[1] = CreateWindowEx(
    WS_EX_CLIENTEDGE,
    "Child Window Class Name",
    NULL, 
    WS_CHILD | WS_VISIBLE,
    0,
    rectView.bottom/2+1,
    rectView.right,
    rectView.bottom/2-1,
    hwnd,
    NULL,
    hInst,
    NULL
   );
   boundary = rectView.bottom/2;
   hCursor = LoadCursor(NULL, MAKEINTRESOURCE(IDC_SIZENS));
   return 0;
  case ID_EXIT :
   PostQuitMessage(0);
   return 0 ;
  }
  return 0;
 case WM_MOUSEMOVE:
  if (HIWORD(lParam)>=boundary-2 && HIWORD(lParam)<=boundary+2) SetCursor(hCursor);
  if (wParam == MK_LBUTTON && split == TRUE)
  {
   GetClientRect(hwnd, &rectView);
   if (rectView.top+5 < HIWORD(lParam) && HIWORD(lParam) < rectView.bottom-5)
    boundary = HIWORD(lParam);
   MoveWindow(ChildHwnd[0], 0, 0, rectView.right, boundary - 1, TRUE);
   MoveWindow(ChildHwnd[1], 0, boundary+1, rectView.right, rectView.bottom - boundary + 1, TRUE);
  }
  return 0;
 case WM_LBUTTONDOWN:
  if (split == TRUE)
  {
   SetCursor(hCursor);
   SetCapture(hwnd);
  }
  return 0;
 case WM_LBUTTONUP:
  if (split == TRUE) ReleaseCapture();
  return 0;
 case WM_DESTROY:
  PostQuitMessage(0);
  return 0;
 }
 return DefWindowProc(hwnd, iMsg, wParam, lParam);
}

LRESULT CALLBACK ChildWndProc(HWND hwnd, UINT iMsg,
         WPARAM wParam, LPARAM lParam)
{
 HDC hdc;
 static int x[2] = {20, 20}, y[2]={20, 20}, flag[2];
 int select;

 switch (iMsg)
 {
 case WM_CREATE: 
  break;
 case WM_TIMER:
  x[wParam] = x[wParam] + 20;
  hdc = GetDC(hwnd);
  Ellipse(hdc, x[wParam] - 20, y[wParam] - 20,
    x[wParam] + 20, y[wParam] + 20);
  ReleaseDC(hwnd, hdc);
  break;
 case WM_LBUTTONDOWN:
  if (hwnd == ChildHwnd[0])
   select = 0;
  else
   select = 1;
  flag[select] = 1 - flag[select];
  if (flag[select])
   SetTimer(hwnd, select, 100, NULL);
  else
   KillTimer(hwnd, select);
  break;
 case WM_DESTROY:
  return 0;
 }
 return DefMDIChildProc(hwnd, iMsg, wParam, lParam);
}

예제 그대로 긁어옵니다
어떻게 위에 자식윈도우를 누르면 위에건줄알고 동그라미가 쭉생기나여?
스샷은 위에자식윈도우에 클릭하고 그담에 아래자식윈도우 클릭한거임