@Dezaki parallel will was supposed to be used to cast magic that needs a squad of mages by oneself, they weren't supposed to leave it on all the time as it's like multiple personality disorder.
@Memi Well, it's not that you can't, but it's as you said like Multiple Personality Disorder amped up to eleven.
Shun's teacher does it I think, and he said that he can use it but that eventually the wills start arguing about who was the original and things like that. It's wasn't clear if Shiro was immune because she's a spider or because she's super pragmatic, because if you remember she divided up the wills to different roles in the body.
Also mother spiders are known to eat their lovers and their children because it takes a lot of energy to... Procreate cough it's also known that mother spiders save some of their young after hatching to eat at a later date as... A backup meal if you will. Not all species but I believe black widows do this
@unime It is true, but in this case mother I believe she has with Kumoko. xD
Look at the insistence she pursues, I wonder if there is a particular reason, perhaps in the novel it is explained.
If the story of the dragon Alabar has been explained, maybe even this is explained.