Communication with different dialogue box in same MFC dialogue based application

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MFC

VisualC++

I already designed one dialogue based application in MFC and I want to add two dialogues before the main dialogue open.I tried to call one of the new dialogue through OnInitDialogue() function and I do that using DoModal() method in old application,but my condition is that old main dialogue should be opened as some variable conditions and not opened with pressing escape button or closing new dialogue!
and another query is: I used Back button on old dialogue which should display previous dialogue! How to do that?
I copy some code from my application below:

CDialog::OnInitDialog();
	

	// Add "About..." menu item to system menu.

	// IDM_ABOUTBOX must be in the system command range.
	ASSERT((IDM_ABOUTBOX & 0xFFF0) == IDM_ABOUTBOX);
	ASSERT(IDM_ABOUTBOX < 0xF000);

	CMenu* pSysMenu = GetSystemMenu(FALSE);
	if (pSysMenu != NULL)
	{
		CString strAboutMenu;
		strAboutMenu.LoadString(IDS_ABOUTBOX);
		if (!strAboutMenu.IsEmpty())
		{
			pSysMenu->AppendMenu(MF_SEPARATOR);
			pSysMenu->AppendMenu(MF_STRING, IDM_ABOUTBOX, strAboutMenu);
		}
	}

	// Set the icon for this dialog.  The framework does this automatically
	//  when the application's main window is not a dialog
	SetIcon(m_hIcon, TRUE);			// Set big icon
	SetIcon(m_hIcon, FALSE);		// Set small icon

	// TODO: Add extra initialization here
	UpdateData(TRUE);
	CMainApp dlg;
	dlg.DoModal();

and code below explains how I called new dialogue on clicked event of Back button of old dialogue:

void CCheckout_Code::OnBnClickedBack()
{
	// TODO: Add your control notification handler code here
	UpdateData(TRUE);
	CPrevApp startDlg;
		startDlg.DoModal();
}