And an inoperable set of Siamese Twins fails your personal definition of humanity as well, as they are not both an "indivisible lifeform" and "an individual", right?
You want to be able to define human life and defy all other definitions. Your hubris is amazing.+
By indivisible, I mean that you cannot take one human being and divide it into two. Siamese twins are the result of a zygote not dividing fully. The zygote was not two human beings at conception. If anything, your example supports my point that life does NOT begin at conception, but begins at some other point in the gestation process.