def _interleave_digits(first_digit, second_digit):
    """
    Interleaves both 2 of 5 encoded digits by creating
    a composite string that represents the interleaved
    version of them.

    This process is specific for the general version of
    the 2 of 5 true type font encoding.

    :type first_digit: String
    :param first_digit: The first digit encoded in the
    2 of 5 normal encoding.
    :type second_digit: String
    :param second_digit: The second digit encoded in the
    2 of 5 normal encoding.
    :rtype: String
    :return: The interleaved representation of the concatenation
    of both digits in the normal 2 of 5 barcode format.
    """
    interleaved_leetters = []
    for index in range(5):
        first_letter = first_digit[index]
        second_letter = second_digit[index]
        second_letter_lower = second_letter.lower()
        interleaved_letters = first_letter + second_letter_lower
        interleaved_leetters.append(interleaved_letters)
    interleaved_digits_string = ''.join(interleaved_leetters)
    return interleaved_digits_string
