Are you arguing that not all twins have a soul? Or are you just making an atheist argument against souls altogether?
Monozygotic twins, frequently referred to as identical twins, occur when a single egg is fertilized to form one zygote (monozygotic) which then divides into two separate embryos. The division of the developing zygote into 2 embryos occurs within 8 days of fertilization 99% of the time.
Not arguing anything, just seeking opinion. It is a simple question, actually. If life begins at conception, and there is no conception, is it life? If this irritates you, I am sorry. But perhaps I am the canary in the coal mine. The question is not going to go away. There have to be many, many folks asking the same question, and the church is going to have to answer, eventually. Maybe not today, or tomorrow, but it is not going away.
I believe in God with all my heart. Peace.