What is the ruling on marrying secretly without the first wife knowing?

Shaykh Ṣāliḥ al-Fawzān:

She has no connection [authority]. The [first] wife has no authority to prevent her husband [from marrying] or to make it a condition upon him that she is told [about a second marriage]. She should not make such a condition. The important factor [in polygamy] is justice. The husband is obliged to treat [all] his wives equally, whether the first wife knows [about the second] or not.