A Chimera can be formed from two blastocysts at least up to at least 7 days after fertilization, and probably as late as 13 days.
Twins can form from one embryo up to 15 days (although after 13 days they will probably be conjoined.
You know, I just asked this exact same question of one of my extreme right-to-life colleagues on another forum. And her response was to completely ignore the question and shriek, "You're willing to risk killing another person based on your own fallible human 'sense'?!"
So I guess that's your answer.
The problem of twins and chimeras was one of the things that led me to abandon the conception position (my husband is an identical twin). The other major factor was the fact that 30%-70% of blastocysts fail to implant all by their lonesome. If human ensoulment takes place at conception, then 30%-70% of the human race is passing into the afterlife without ever making it past the implantation phase, all because God designed our bodies that way.
That was a lot more monstrous and problematic to me than the prospect that I might be killing my own children with birth control pills.
I prefer the life begins at blood
interpretation. It has much more synergy with Christian mystery and much better logical, theological, and biblical support than the conception position. It allows for the possibility that embryonic stem cell research is an option (the jury is still out for me on that one), and IUDs and hormonal birth control are all acceptable under it.
I am what I consider to be moderately pro-life though. I don't believe there's much point in making abortion illegal if we don't treat the reasons why women are seeking them in the first place.
The extreme right-to-life position is just not compatible with observed human development. Adherents do more damage to their position than not - because their position is nonsense, the extreme pro-choice position makes more ground.
I try telling them that. I never get very far past the screaming about murder.