Мой вариант:
Each shareholder of a company being re-organized in the form of division shall retain the same percentage of stock ownership to the total quantity of the stock issued by the newly-emerging companies, being equal to being equal to the percentage of the stock owned by such a shareholder to the total quantity of the stock issued by the re-organized company, unless otherwise is provided for by the charter of the company.

Заранее спасибо!

Unless otherwise provided for by the company's charter,... (так фраза выглядит компактнее). Это можно и в начале поставить (через запятую). Заметили, что я "is" выбросил?

...the same number of its (или his/her) shares in percentage to the total number of issued shares of the newly formed companies as the number of its shares in percentage to the total number of issued shares of the reorganized joint-stock company.