See the 1st answer to this question. In summary, the combination note is a substitute for a zakef katan. Apparently, the tune is similar to it, but not quite. I'm assuming that those readers that make a distinct tune for this trope combination may be attempting to "imitate" the zakef katan, but make it sound a bit differently, as it is not exactly a zakef katan.
Offhand, I don't think it requires a distinct sounding that wouldn't otherwise be done if these two notes were on separate words, except that it's done on 1 word. After all, there are numerous places where we see notes (such as kadma v'azla) on a single word, and we don't sing it differently than if it were on 2 words.