From the documentation for mb_detect_order, the function that establishes the order in which mb_detect_encoding
tests different encodings:
mbstring currently implements the following encoding detection filters. If there is an invalid byte sequence for the following encodings, encoding detection will fail. UTF-8, UTF-7, ASCII, EUC-JP,SJIS, eucJP-win, SJIS-win, JIS, ISO-2022-JP
For ISO-8859-*, mbstring always detects as ISO-8859-*.
For UTF-16, UTF-32, UCS2 and UCS4, encoding detection will fail always.
So, you can't detect the encoding of the second string with the mb functions.