We only add an extra day for a D’oraisa like the ‘Sholosh Regalim’. Chanukah is a D’Rabbanan. (See Taamei Minhagim 864 in the name of the Avudraham)
In addition as Chanuka starts on the 25th day of the Hebrew month we can presume that the Sheluchei Beis Din would ofhave arrived and everyone would know when it actually began.