When does soul begin?

Some say the soul begins when a cell is fertilized. But what about identical twins, Siamese twins, and chimera?

Of course, identical twins and Siamese twins start out as a single fertilized cell. So, according to this theory, they start out with a single soul. But surely each identical twin has its own soul. (I'm using "its" to avoid boring repetitions of his/her).

Chimera start out as fraternal twins whose cells merge, so that the person has cells that originated from two separate fertilized eggs. So, there are two souls to start out with, for what turns out to be a single individual.

Siamese twins start out as a single fertilized cell, which goes part-way to being identical twins. There may be two mostly separate bodies sharing a small band of tissues or sharing major organs. There might be mostly one body with one head and more than two arms. And various other combinations. Are there one or two souls?

It should be possible for a chimera to end up being Siamese or semi-identical twins, but I think we already have enough options to show the problems.

My feeling is that there is a soul for each distinct brain. I wonder what other people might think/feel.