(n) a word design that reads one word when upright and another word when upside-down. Whereas an ambigram
reads the same word both upright and upside-down, a symbiotogram presents a different text in the inverted orientation.
The "Illuminati Diamond" from the Angels & Demons book is an example of a symbiotogram as are several of the examples on John Langdon's web site. The home page of FlipScript ambigram products shows a symbiotogram of two names.