It appears to be a question of appropriateness, and Goddess knows, we love inappropriate things. However the difference in the definitions of Syllepsis and Zeugma, has to do with how appropriate the application of the one word is to the two. Syllepsis is defined as a Rhetorical Device in which one word, or possibly phrase, is used toward two objects, as in the sample phrase: "She went to that place where angels fear to tread, and shopping." [Where "went" is the one word, and "that place where angels fear to tread" and "shopping" are the two concepts]. If this formula is used but the 'one word' is, at least seemingly, impossible to make work with one of the 'two words', then the Rhetorical Device is considered to be a Zeugma, though this is often intentional to produce some particular affect upon the hearers and/or readers. A sample of a Zeugma might be: "He had an esoteric belief system, and hat" [where "esoteric" is the one word, and "belief system" and "hat" are the two things or concepts]. Of course, as metaphor broadens, the distinction becomes more subjective, and the definitions less concrete. Often an attempt to make the two words or phrases represent as disparate a combination of things and/or concepts as possible, without making application of the 'one word' to them impossible, seems to increase the perceived success of the Syllepsis. Intentional use of Zeugma is often an attempt by the originator to startle, and/or attempt to push the intended audience into new pathways of thought, in reference to the utilized subjects.