Thank you for your question. Adultery is when a couple have relations outside of a valid contract of marriage. Since informing the first wife of the marriage contract with the second wife is not a condition for its validity, the intimacy with the second wife is not considered adultery. Polygamy is allowed in Islam and is not considered immoral or sinful in the eyes of God. At the same time, a man should be wise in how he builds a home and the feelings of his family members. But there is no doubt that by taking a second wife without the knowledge of the first is not a sinful act.
May you always be successful