Dual codes are too indeterminate
Dual codes are inadequate as they're too ambiguous to provide for correspondences between pictures and words. Moving from pictures to words and vice versa requires an underlying intermediary code, or interlingua, to mediate the two realms.
For example, someone shown a picture of some unchained picture of a rose doesn’t know whether the word to associate with it is "rose", "flower", "plant", or another word.
Zenon Pylyshyn (1973).