Assuming the parents want them to be together - Taking away that security age 4- when leaving their mother all day and starting in a new school- why?
I have an older daughter - she didn’t need or want a security blanket when she started school, she made friends quite happily.
The same principle when my twins started school, separate classes so they could make friends, which they did.
I think t some twins parents enjoy the fact they have twins and want them as a couple.
They have to separate at some point college university leaving home, why not give them the independent skills when young?
I know a fair few sets - those who’s parents have insisted they are together for everything have one twin who’s struggling and just follows.
I know twin parents who kick off if one is invited to a party and the other hasn’t been - that’s life for all siblings.
I know twin moms who’s one twin has made a friend and insists the other twin is included in everything -
Let them be their own person, no need for a security blanket.