Private Sub picSplitMain_MouseDown(Button As Integer, Shift As Integer, x As Single, y As Single)
  If Button = vbLeftButton Then 
    HookSplitter picSplitMain.hwnd
  End If
  DoEvents
  m_cSplitMain.MouseDown Button, Shift, x, y
End Sub

Public Sub HookSplitter(gHW_ As Long)
  If IsHooked Then Exit Sub '
  IsHooked = True
  'Start subclassing.
  gHW = gHW_
  OldSplitFWindowProc = SetWindowLong(gHW, GWL_WNDPROC, _
   AddressOf NewSplitFWindowProc)
End Sub

Public Sub UnhookSplitter()
  Dim temp As Long

  'Cease subclassing.
  temp = SetWindowLong(gHW, GWL_WNDPROC, OldSplitFWindowProc)
  IsHooked = False
End Sub

Function NewSplitFWindowProc(ByVal hwnd As Long, ByVal Msg As Long, _
 ByVal wParam As Long, ByVal lParam As Long) As Long

  If Msg = WM_CAPTURECHANGED Then
    Call UnhookSplitter
    m_splitter.MouseUp 0, 0, 0, 0
  Else
    NewSplitFWindowProc = CallWindowProc(OldSplitFWindowProc, hwnd, Msg, _
     wParam, lParam)
  End If
End Function

Public Class Form1
  Dim WithEvents s_Splitter1 As SubclassMyHWND
  Dim IsSplitter1Hooked As Boolean 'применен хук

  Private Sub s_Splitter1_CallBackWndProc(ByRef m As Message, ByRef Cancel As Boolean) Handles s_Splitter1.CallBackWndProc
    Debug.WriteLine(m.ToString)
    If m.Msg = WM_CAPTURECHANGED Then
      s_Splitter1.UnSubclass()
      IsSplitter1Hooked = False
      SendMessage(Me.Splitter1.Handle, WM_LBUTTONUP, 0, 0)
      Panel2.Refresh()
      Cancel = True
    End If
  End Sub

  Private Sub Splitter2_MouseDown(sender As Object, e As MouseEventArgs) Handles Splitter1.MouseDown
    If e.Button = Windows.Forms.MouseButtons.Left Then 'только левая кнопка мыши
      If IsSplitter1Hooked Then Exit Sub
      IsSplitter1Hooked = True
      s_Splitter1 = New SubclassMyHWND
      s_Splitter1.Subclass(Splitter1.Handle)
    End If
  End Sub
End Class

  Private Sub Splitter1_MouseCaptureChanged(sender As Object, e As EventArgs) Handles Splitter1.MouseCaptureChanged
    SendMessage(Me.Splitter1.Handle, WM_LBUTTONUP, 0, 0)
    Panel2.Refresh()
  End Sub