how to get the handle of a TextBox of one Dialog in another Dilog's sourcecode?

If you have the handle to the dialog, you can simply use a SendMessage() call and return the handle cast as an LRESULT.

Using pseudo-code:

//in target dialog (message catcher) include the message id
#define MSG_GETHANDLE (WM_APP+1)
//...include in message map
BEGIN_MESSAGE_MAP( TargetDialog, CDialog )
	ON_MESSAGE( MSG_GETHANDLE, OnGetHandle)
END_MESSAGE_MAP()
//...define method
LRESULT TargetDialog::OnGetHandle(WPARAM,LPARAM)
{
  ...
  return (LRESULT) handleTextBox;
}

//Within your second dialog, just send the request message and interpret response
CEdit *textbox = (CEdit *)TargetDialog->SendMessage(MSG_GETHANDLE,0,0);
//now you can use textbox

I should say however, that it can be considered bad practice to write directly to another dialog's internal controls. A better method would be to send a request for data or for a change in the GUI and let the parent dialog change the controls.