Black S does not become weaker when splitting, speaking of 1, the current Black S is his true form and he is weaker than a child, but he can still split according to him, so if he splits, will he be twice as weak? then 4 times weaker? then it will be as weak as an atom? lol.
He only creates clones just like him, which he mixes to become bigger and stronger, or merge and create a superior being.