Question:

If a person wants to get married to more than one wife does he have to seek permission from the first wife? Or is it simply that he has to be just?

Answer:

It is not necessary to seek permission from the first wife, however, it is best to. It is necessary to be completely just between the wives. If a person has the fear that he will be unjust in the slightest manner then it is not permissible to marry a second wife.

And Allah Subhaanahu wa Ta’ala knows best

(Hazrat Mufti) Muhmmad Taqi Usmani (May Allah preserve him)
Fatawa Usmani Vol.2 Pg.309