In medical circles can refer to twin reversed arterial perfusion, or a subset of conjoint (siamese) twins. Two fetusii develop conjoined, but one (the parasite) does not fully develop and is wholly dependent on his host brother.
In team management refers to a redundant slacker who performs an identical, complete subset of the effort performed by a more capable worker.
a: we have the same job! we're like siamese twins!
b: nah, I do twice as much. you're a parasitic twin