def _non_join(string1, string2):
    """Joins two strings if they are not ``None``.
    
    Returns ``None`` if either of the input strings is ``None``.
    
    Parameters
    ----------
    string1 : str 
        First string
    string2 : str 
        Second string
    
    """
    if string1 is not None and string2 is not None:
        return string1 + string2
    else:
        return None
