Put the message map into a separate .h file and #include
it into both classes.
BEGIN_MESSAGE_MAP(CMyPreviewView, CPreviewView)
//{{AFX_MSG_MAP(CMyPreviewView)
ON_COMMAND(AFX_ID_PREVIEW_CLOSE, OnPreviewClose)
ON_COMMAND(AFX_ID_PREVIEW_PRINT, OnPreviewPrint)
ON_UPDATE_COMMAND_UI(ID_WINDOW_NEW, OnUpdateWindowNew)
#include "SharedViewMessageMap.h"
//}}AFX_MSG_MAP
END_MESSAGE_MAP()